I had this problem too and it drove me crazy.

Before you do what Dazz is suggesting try bending the loop on the top of the ring forward (towards the mast track). This will help it hook, but don't go too far or you won't get it unhooked. You need to do a bit of trial and error but once it's right life is good again.

The above is assuming the knot on the top of the ring is facing aft.

The reason why some mains are hard to hook and others aren't is because the headboard isn't in the exact same location relative to the hook from sail to sail. So, if you have multiple mains you may need to adjust the ring/loop again. Or have separate rings... my inventory isn't that deep :-)