This is a common one... and the reason a lot of trainee lash artists quit before they've even started!
It's important to know that you are not the problem. The lashes on dummy heads are made of a plastic material very unlike a human lash - this makes it hard for the lash extension glue to attach to it.
My top tip for lashing your dummy is to prime the dummy lashes (this creates something for the glue to stick to)