Can't visit friend's towns despite them updating

I can now visit approx 50% of my 'friends', but there are still some who have updated that I can't visit. Of those that I can, only around 50% ever have any cards to select, & of those cards I only get a heart in about one of five. If I go through all the towns I can visit (30ish) I'll only manage to get about 7-8 hearts. A lot of time/effort for very little return.