hmmmmmmmmm......... that is weird.
They snagged pretty easy for me. I would try to put them to sleep if at all possible. Make sure that the HP is in the red or yellow.
And just keep at it. Yes I know it is very frustrating when they dont snag. Trust me I have been there. Paralyzing them worked for me.
But I think putting them to sleep is your best bet.
Good luck!!!!!!!!!!!