To my knowledge (and this is something that I haven't researched extensively), the HK45 magazine spring and the USP45 magazine springs would not be compatible due to size and capacity differences in the magazines. The HK45 and USP45C 10 round magazine spring should work though.
All this being said, check your grip first. Grip is the #1 cause of failure to lock back on HK pistols.
"Throughout history, it has been the inaction of those who could have acted; the indifference of those who should have known better; the silence of the voice of justice when it mattered most; that has made it possible for evil to triumph."
-Haile Selassie I