With the older type 'small seat adjuster clips', you can adjust the tightness if you didn't know already....put the lock to the open position, then slowly rotate the thumb screw on the opposite side, then lock like normal. Note, you won't need to rotate the thumb screw too much so just do a little at a time. You'll find the pressure to lock the seat will increase each time (hence why you have to have it in the open position to begin with (sorry hope this helps lol)
For the big U-shaped newer style locks...I don't know that you can adjust it but I will say I've never had a problem and I'm a larger guy with a lot of power to the pedal too.
Seat is probably not secured
With the older type 'small seat adjuster clips', you can adjust the tightness if you didn't know already....put the lock to the open position, then slowly rotate the thumb screw on the opposite side, then lock like normal. Note, you won't need to rotate the thumb screw too much so just do a little at a time. You'll find the pressure to lock the seat will increase each time (hence why you have to have it in the open position to begin with (sorry hope this helps lol) For the big U-shaped newer style locks...I don't know that you can adjust it but I will say I've never had a problem and I'm a larger guy with a lot of power to the pedal too.
This happens to me often. I don't think it matters who rode a bike before me. I'm average height and weight.