Trade doesn't work

Sorry if my english isn’t very good, I hope you will fully understand this message

Hello, I wanted to trade my delta scyther with a friend to make it evolve.
He received my delta scyther, and I received his axew. My delta scyther has evolved into a delta scizor. We were able to trade the first time.
Now we want make an another trade because I want to get back my delta scizor (and he wants to get back his axew) but trade doesn’t work anymore.
We aren’t on the same IP, we aren’t on challenge mode, etc
So well, I want to know why we can’t trade and how we could be able to re-trade.
Thank you !

Edit : finally my friend put my pokemon on GTS.
I find it but when I try to trade my game crash. Any idea ?

Edit 2 : I trade my scizor against my friend’s pokemon. He received his axew and he doesn’t have my scizor anymore but I still have his axew too.
Did my scioz was transformed into a little axew ?
Do I have a method to get it back ? D:

Afaik, GTS is extremely bugged to the point that you can’t withdraw the pokemon deposited.
Your best bet would be for your friend to load a backup save and then trading the delta scizor to you. It’s likely that the trade didn’t complete on your end because of a momentary disconnect from the server, hence your friend receiving the axew while you still have it.

Oh well, what a shame, but it doesn’t matter a lot.
Finally ma friend trade his own delta scyther, and it replaces my own scizor, I make it evolved thank to the npc in the black market.
Thank you for your help, have a nice day !