Dunno if J's tried this, but I've heard that doing a bunch of her chores for her does wonders for a woman's libido.
And it could be a communication (not enough verbal/emotional intimacy) problem. Or like Snowbunny mentioned, could be that J's not that wound up about it, and not inclined to search out possible ways to get A interested. Do A and J have date nights? Just wondering.
A should probably flat-out ask J if this is a problem for him, and if it is, then A and J ought to start seeing a (preferably poly-friendly) counselor. Possibly a sex counselor, depending on what the problem turns out to be.
In a nutshell, if you want to make someone feel special, you have to do special things for them. But A needs to exert a little faith and effort too -- if this is a problem for J. Something for A to investigate, I think. Nothing blocks the solution to a problem quite like assumptions do. So first order of business is an in-depth discussion about it between A and J.
Having said all this, it occurs to me that this is really something that A and J need to work out between the two of them. Your place is to be a good friend and lover toward A, and a good friend toward J. Their bedroom issues are really out of scope of what's appropriate for you to try to fix. If A asks for your counsel, perhaps you might direct her to this thread or something along those lines.
Hope things continue to go well.