ian beat me to it, but he asked the right questions.
is it a full disc or a pucked clutch? pucked clutches do not require break-in when new, but even if it was a full disc, its used = no break in. i'd definitely play with the pedal adjustment, you might just not be having full engagement, or perhaps you need to bleed the clutch line if there's any air in there it would make adjusting the pedal useless.
also did u just slap it on the stock flywheel that was in there? the flywheel might need to be resurfaced (turned). it's not necessary being with a used clutch, but it might have some wear and small grooves in it from the prior clutch that may be preventing the new (used) one from grabbing properly.
i will leave this open seeing as you guys seem to be working it out. if it needs to be deleted later on, so be it.
btw - a shop is truly judged not on it's initial business, but it's ability to fix a customer's problems. just be polite and professional about things, im sure ian can help you and maybe check it out for you.