Hi Kenton, a few things to consider (before throwing the HT under the bus)…
This could be normal… The only way the RH inside the HT is really going to drop is with the desiccant in there sucking out moisture. The filament may still be releasing moisture, which is the point, and that air is being exchanged with the outside air and in theory being removed, but in exchange it’s just sucking in ambient air. And with the rather minuscule amount of desiccant in the HT, only so much moisture can be removed before the air is exchanged. In contrast, the large AMS units can hold a lot of desiccant, especially if one adds extra storage bins.
Getting extra desiccant into the HT, eg. in a container mounted in the center of the spool being dried, can definitely help.
The filament could already have been dry, or as dry as the HT will ever get it in the current environment, so there’s not going to be any observable RH change.
Now for the issues… folks here have posted varying experiences with the HT, so definitely YMMV (there are threads on here worth reading). Personally I gave up on the HT as a dryer, but I’m trying not to let that influence my advice too much.
One thing reported is the inaccuracy of the humidity sensor, which for at least some folks reads high (possibly due to its position within the unit, sometimes due to packing tape still being left during assembly, covering the sensor… lol). The humidity towards the back of the unit also tends to be lower (where the exhaust vent and desiccant packets are).
Another thing is the venting itself, which may not be entirely effective. For example myself and others have better results if we briefly opened the HT once in a while to release trapped moisture… the humidity reading can drop several percent in some cases (then slowly build back up). The vents themselves have also known to fail or be defective (and not open/close correctly), which should and in most cases does throw an error at you, but apparently not always (can be visually checked).
So is the filament getting dried? IMHO the simplest and most direct way to check is to print with it. Another common measure besides monitoring environmental RH is to weigh the spool (on a scale with at least 1g resolution) before drying and then compare to weight after/during drying. It’s still a relative metric (doesn’t tell you how much moisture is actually in there), but in theory once it stops loosing weight then it’s as dry as the drying solution can get it. That doesn’t measure ultimately how effective the drying was, but it’s something, and quick to do.
HTH,
-Max