what's the trade message you get?
if it's that your trade was cancelled because a pet was moved or deleted, it's because someone else asked for a pet you wanted first, and the OP is accepting trades in the order they received them. when your trade is cancelled because the user accepted another trade first, it's called "auto-cancelling"; posts like that usually get lots of attention, so a lot of people get their trades auto-cancelled. most users giving away free pets are open to you sending another trade if that happens!
edit i'm not super well versed with forum stuff but
this may be a better place to post next time if you have any other questions!