It's very likely that your basses are reacting to the dry indoor air brought on by the winter heating season.
Do an online search for "homemade guitar case humidifier." Make a couple and put them in your bass cases. Store your basses in the cases when you're not playing them. I do this.
The necks on my basses stay very stable. I haven't yet had to tweak the necks on any of my basses since setting them up to my liking. My Birdsong Skyrider is in its fourth winter in Wisconsin and doing fine.
I use cellulose sponges in plastic sandwich bags. Poke a bunch of holes in the bag. Soak the sponge in water, squeeze it out thoroughly, put it in the baggie. Throw it in the case. Check the sponge periodically and remoisten as needed. If the bass necks are very dried out, it may take several weeks for them to get to the proper moisture level. The goal should be to maintain a humidity of 45% - 55% in the case.
Birdsong Skyrider 9K-001P, Birdsong Sadhana 10S-006, Aria Pro II Cardinal CSB-450, Gretsch Electromatic Jr., Squier VM Jag SS (red!), Yamaha BB300, Univox Hi-Flier, Birdsong Club Member #33