did you try running each fan individually off the same molex ? did they both work separately ? did you try a different molex connector ? is the PS a piece of junk ? have you tried another fan........ any size fan and try and run them together off the same molex ?
It's probably a defective fan but here are some troubleshooting steps. Disconnect the front fan and see if the top fan starts consistently. You can also try to move the blades once the PC is on. Fans require more power to start and a little nudge might help.
Sounds like it's dying. Fan's die a slow death. Typically they start to barley spin, if there are LED's, they will fail to light up progressively or collectively dim. You might even smell an odd smell from it.
Just get a new one to replace it. They're typically between $3-$10.