I also tried for spider and got my 4th diamond! Hermes/Goblin. I feel like once you've finally bred a dragon that you want, it becomes far more likely that you will get another one. However, this also seems to make it far more difficult to breed new ones.
Now I've got diamond tying up the breeding den while I'm trying to breed a limited triple type! I think I will speed it up tonight (even though I hate using that much gold) so I can have a better chance for Spider