Assuming this is Windows and not any other OS, but without make and model computer, or any other information, I'm just guessing that something is interrupting the screen saver.
Any programs running? Anything running in the background? Any file sharing going on? Is the computer on a home network?
Look for something that s interrupting the screen saver.
Today I noticed that my screensaver won't stay on. I have it set to activate after 10 minutes and it goes on but immediately goes off. It is one of several I have that are activated scenery and play music. Up puntil now they have been working just fine.