often on older steel bikes
the two ears on the back of the seat tube
through which the bolt runs
get slightly deformed
and contact eachother and stop the seatpost from being further tightened

to solve this problem
take a file or hacksaw blade to the inside edge of the ears
so they have more room to tighten

also
most seatpost binder bolts for older bikes
are keyed
and the key fits into a keyway in the hole in the ears
so you only need one wrench to adjust