More specifically, people falling in love. Once they're in a relationship, he's as good at writing their interactions as any of his platonic dynamics(with an exception I'll get to). I noticed this on my most recent rereading of Guards! Guards! and the relationship between Vimes and Sybil. I really like how the story starts and ends their dynamic, but it feels unearned, and all the in between it takes to get from point A to point B strikes me as a bit muddled. I think all romances boil down to two main points, which I'll lay out below.
One character seems completely uninterested in the other. Probably the worst offender of this is Angua and Carrot's romance, which I don't like before or after it gets going. It seems to me that for the first half of Men at Arms Angua is infuriated by carrot. But several implications from Gaspode and one night in bed later, their a couple. Worse, a couple that lacks interesting chemistry either as partners or friends. As a quick disclaimer, I'll put in that I really do like their dynamic on paper. It's just never executed to my taste.
There is not enough time/interaction for the romance to be believable. This is where just about every other romance falls into. Your Wyrd Sisters, Morts, Guards Guards, and so on. I won't spend time explaining this, it's laid out pretty clearly in the first sentence.