I wonder if it was just the two I read being weaker - Moving Pictures (great concept, very on point but didn’t really enjoy it)
I think that might have been a bad choice. I read that almost 30 years ago and abandoned about 20 pages before the end, something I don't think I've ever done with any other book. I've read other Discworld books since that I've quite enjoyed, even if they didn't knock my socks off.
I think the problem for me with Thief of Time was that it jumps around a lot at the beginning, and has a premise that is both quite complicated and doesn't really interest me. I think it's also quite heavy on the "forced whimsy" feeling that isn't so prevalent in some of his others.