I can think of several reasons that she might not be attaching on that side, There are things you can do about it, but in the meantime, please don't worry as your supply on the left side would be compensating.

She might have difficulty for a physical reason, say as a result of the birth or her positioning in the womb. (Sometimes a paed chiropractor or osteopath can help). You could try putting her in a different hold on that side, to make sure she isn't uncomfortable. So try the football hold if until now you've been trying the cradle hold. Feeding her lying down on that side will possibly also help.

Your letdown might be slower on that side. You can try speeding it up by expressing a little off that side first, to ensure you've had a letdown. Or put her on that side first, switch when she won't stay on and go back to it after you've finished on the other side. Try to relax before offering her this side, if you tense up it may be affecting your let down. Make sure you visibly relax your shoulders, take deep breaths and don't stress if it doesn't work.

If you'd like to build up the supply on the right side, then try expressing after you've fed her if she hasn't taken anything/much off that side.

Have you seen a LC? It can help to have someone there to watch the bf closely and perhaps make some suggestions to help her attach. Alternatively, if you can get in touch with the ABA, find your local group and pop along to their next meeting (you might even be able to see a counsellor in your area). Seeing them in person can often make it easier to figure things out.

GL and well done for persisting despite your troubles. Not everyone 'enjoys' bfing, hun, but you should be proud of yourself for the effort you are making to get this right.