If a retailer has sold you a #10 spey line for a pike rod you should go back and rip his head off, totally unsuitable, designed for entirely different purposes, and totally unprofessional. A #10 line is not the same as another #10 line just because they both say #10 on the box, in this case, they're staggeringly different.
A #10 pike line is short, 25'-30' typically, as you're not casting a long way, is for a shortish rod, and has heavy front taper for turning over big flies.
The spey line is much much longer, 55' to 90' typically, and is designed for spey ( roll ) casting using long rods with most of the weight at the rear, with an average to small taper at the front for normal/small flies. I'm not surprised you're struggling to cast it, with 10' out on some spey lines that's still the forward taper!
Which retailer was this? It really is shockingingly unprofessional, either they've deliberately sold you a pup, or they have no idea what they're doing, either way people should know.

God this unprofessional **** annoys me, they have no right selling stuff if they don't understand what they're selling!