If you sus that the rim is bent, detension the spokes completely and see if there's a significant bend. If so, you may never be able to get it true. I've heard people say that you can sometimes straighten them out by laying it flat on the ground and pounding out the bends witha rubber mallet. It may be so but the one time I tried it didn't go so well.

Otherwise joints can be a hassle on lower quality rims but you should be able to get it true. Your tension around the joint may not be ideal.

How are you checking your tension?