It's an okay quality but low end ("workhorse") pistol.
Yours is defective. Take it back and get an exchange/refund. If you fuck with it yourself you'll probably void any returns and it'll end up costing you $100+ for parts (and maybe labour) to get it fixed. If you're gonna drop an extra $100 into it right out the gate might as well get a higher end product.
As for the slide not locking back, if you're shooting outside when its cold then yeah, a stock gas gun running propane/green gas will not perform that well.