Dan old boy (or young boy),
A couple good pix's would help us give you a better answer .. I live over here in wet, damp, rainy Western Washington. My Flow-hive back door swells at times also. I used a wood rasp to reduce sticking most not not all the time. Around here atmospheric moisture going from kind of dry to soaking WET in few hours. I don't sweat the small stuff. I just keep a small old flat blade screwdriver handy to ply when the back doors fingers swell too much. Not a big deal !
Unfortunately it is the nature of all woods to shrink, swell, twist n warp. I have a small wood shop n store my more expensive woods at a certain humidity. The bummer is our pine n cedar hives are out in the full envirment 24/7 n 365 days a year. Also our bees give off humidity inside as well. That's a tough one for most woods. Some of the hive bodies I make personally I glue 3 or 4"
wide pieces to get the width .. That often helps with the warping but not swelling n shrinkage. Each Flow-hive cedar board is cut from different section n part of the log thus some have more problems with cracking n warping. thats the nature of woods. Guess I'm lucky so far. Four of my 8 hives are cedar.. It's rot resistant n light for this old bugger to lift n move without killing my back.
"If" cracking (others have given good suggestion already). occurs or knots get loss you might try a good CA epoxy glue. I turn woods on a lathe with check, knots n few cracks as well as bad/soft spots. I'd try a CA product but apply from the inside as the outer Tung or paint is on the outer side of your hive boards. Here couple pics of the glue n bowls I have turned. This special CA hold n supports longer life to board. I've treated all my Flow-hive knots with this. I'm
lucky as so far I have had only minimum warping n cracking in my second season here.
Dan ! Hope my thots help a little. Ohh ! Still post us a few good pics. (Close up n full) so we can see n check !
Good luck n happy beekeeping.