Likely undertorqued as mentioned. If you don't have a lot of experience wrenching...even if you do...I generally use a torque wrench on cranks...invest in a good 3/8" drive torque wrench. Doesn't have to be high end wrench like Snapon...Craftsman will suffice. Also good to own a set of socket allens....hex allen bits attached to socket which plug into a torque wrench. Sometimes torque target isn't achievable with L shaped allen due to lack of leverage...or owner doesn't know the torque target. Your post is unclear...you may mean the single center crank bolt...depends on the crank...but if you are refering to left crank arm dual pinch bolts aka Shimano...torque in thirds...set torque wrench 1/3, 2/3 and full torque spec and alternate bolts for uniform clamping. Torque specs are generally found on the web...most crank makers have service manual PDF's on line. I always use a dap of grease on the spline to prevent bonding. Inspite of lubrication, this actually helps clamping strength...grease is captured in micro-voids between male and female splines improving clamp strength.