I would completely disassemble and thoroughly clean the mag, especially if it is new. Lube lightly; you don't want to risk de-activating any primers with excess oil. Then load 'em up and store them that way for a few weeks. Even if you can load only nine rounds the spring will still relax somewhat, but best results come from loading to capacity.
A magazine speedloader such as those made by HKS will help you load that last round by increasing your leverage; it'll also save your thumb and reduce case damage by allowing you to fully depress that ninth round BEFORE inserting that last one. You might want to check the mag lips for roughness; lightly stone off any burrs you find.