The first thing I'd check is the hygrometer. If it's digital and you've had it for awhile, replace the battery first; then either salt-test or use a Boveda 75% pack (which will give the same result as a salt test). Either way, the hygrometer should read 75% after eight hours or longer sealed into a small airtight container with the damp salt or Boveda.
I generally calibrate a second time, using a Boveda pack at my preferred humidor RH (65%); but that's optional.
Assuming the hygrometer is accurate, the most likely culprit is the ambient relative humidity in your home. Wood is porous, and any wooden humidor that isn't sealed (with polyurethane varnish or something similar) will surrender moisture to drier air surrounding it. I have that problem in winter even though the ambient RH in my home hovers in the mid-thirties most of the time. Be sure that the humidor is not in the path of the extremely dry warm air coming from your furnace; in most homes the best location is actually in or near the kitchen, where steam from cooking adds moisture to the air.
If the RH inside the humidor is very low (for me, anything below 60% even with the recommended amount of beads), you might want to re-season the humidor. Be sure to get the RH higher than normal and keep it there for a few days to allow the Spanish cedar lining to absorb all the moisture it needs. (Store the cigars themselves elsewhere during humidor seasoning. A simple tupperdor will do, or even one or more ziplock bags. Move a container of beads into the cigars' temporary residence, since a dry humidor no doubt means the cigars are dry too.)
Don't forget to check the humidor lid's seal both before and after reseasoning (if reseasoning is indicated). Dry wood shrinks, and that includes the lip that provides the seal. If a paper (dollar-bill) test indicates a leak anywhere around the lid, try covering the lip all the way around with a layer of blue painter's tape; do not use ordinary masking tape, because the stronger adhesive may raise splinters if/when you need to remove and replace the tape. Use a sharp knife or razor blade to trim the tape neatly into the corners, so you don't have a double-layer there that might interfere with properly closing/sealing the humidor. This is a simple way to save a wooden humidor with a seal that has shrunk.
To keep the RH higher during winter, just toss a 60-gram Boveda pack or two (rated for whatever RH you prefer, or even slightly higher) into the humidor; they'll provide extra moisture to keep the wooden humidor itself from drying out again too quickly. A longer-term solution is to add more beads and fill up that empty space with more cigars. The cigars will help to buffer the RH inside the humidor ... and of course you can smoke them!
But don't be surprised if you have to replace/recharge the Bovedas (and remoisten the beads) at least once before winter ends.
It goes without saying, but I'll say it anyway: Don't open the humidor any more often, or for any longer, than necessary. I like to transfer a few cigars at a time to a plastic-box traveldor with a known-good seal and its own humidification device, then leave my desktop humidors closed until I need to refill the traveldor. (A 10- or 15-stick traveldor allows enough variety to choose something to fit my mood.) From the traveldor I sometimes transfer a couple of cigars at a time to a three-finger case (with a Drymistat tube in the center position) so I don't have to open the traveldor as often. (In truth I take out three sticks from the traveldor, put two into the case and smoke the third one immediately. Three cigars are usually enough to keep me happy for a day.)
You may need to repeat the reseasoning and/or add extra humidification again next winter ... or anytime the ambient RH is especially low.
You asked about switching to the 70% beads. I wouldn't do that if you prefer 65% RH and if the low humidity is just a seasonal problem. More beads are always good, of course; even when the humidor is running well, the extra beads will simply help it recover more quickly after you open it or add cigars that are either too dry or too moist.
Incidentally, according to David at Heartfelt Industries, using beads of two different RH "ratings" (such as 65% and 70%) is not a good idea. He explained that the lower-RH beads won't accomplish anything except possibly drying out the higher-RH beads a bit more quickly; once the higher-RH beads lose their moisture, the RH in the humidor will drop to the lower RH, and that could happen quickly if the humidor has a problem holding moisture. Naturally the same would happen if you used 69%, 72% or even 75% Boveda packs to raise the RH, but the Bovedas are ideal as a temporary source of extra moisture while the beads are more "permanent" ... and more expensive. if you keep close watch on the Bovedas you can replace or recharge them to provide primary moisture while the beads serve as a backup during dry weather. After the need for extra humidification ends, you can keep the Bovedas in a sealed ziplock freezer bag until they're needed again, with the beads serving to provide all the moisture your humidor should need.
