Awww, that's so sad, Sue!
Maybe others that have more experience can offer a better answer, but I know with my gang, they can seem as moody as people sometimes. I try to spend time with each of them every day, petting or snuggling with them, but some days some of them are just not receptive to it, sometimes for several days running. Then suddenly, the one that's rebuffing me will jump up on my lap craving attention, and then will practically cling to me for days after. They're odd little creatures.
With Kitty, she sometimes makes herself scarce during the day, going off by herself, but when the kittens are put away for the night, she's all over me, and usually sleeps with me. I think that she must get tired of the kittens being around her all the time, and that when she goes into reclusive mode, she's avoiding them more than avoiding me.
You almost make it sound like it's a fear thing with Bela, though. How long has she been behaving this way? I think what I would do is, try not to force the issue by picking her up or keeping her in a room with you if she doesn't want to be there, but instead, put the kittens in another room and then try to coax Bela by offering her treats, or tempting her with a favourite toy, and then if she comes to you, spend some quality time with just her, even if it's just for a few minutes at a time.
Good luck!
Oh, I've been meaning to ask you, do you keep in touch with the girl who took the other kitten? Rosie, wasn't it? Have you heard how she's doing?