Why doesn't the cheddar MELT??

We tried the first cheddar a little while ago and it was still quite mild. We tried the rest of it recently. It is now sharper and more cheddary, however it still doesn't melt like cheddar. We made toasted cheese and it didn't melt and spread as it should.
I read up a bit, and it seems this can be either because the acidity is to high (not clear to me why this happens, maybe if you leave the milk to ripen too long), or because the moisture content is too high (happens if the curds are too wet when you start pressing). Apparently the most likely issue for the beginner cheese maker is the moisture content. The 3rd cheddar I made was crumblier when it went in to the mold and took more pressure to get it to stick together suggesting the curds were dryer. So I have made a further cheddar to this recipe. The wait is frustrating as you don't get feedback on your changes to a recipe for some time - and it is hard to remember what previous cheeses tasted like. This time I let the curds drain longer than before. If you drain too much you start losing some things you want in the cheese, and can also get to the point where the curds won't stick together. All v tricky.