With the assumption that the bike actually is an I/S and both seats are actually I/S seats, do the following:
1. Measure the distance from the latch on the bike to the bar that the rear of the seat slips into.
2. Measure the distance from hook on the front of each seat the rear hook on each seat.
If both seats measure the same, then this should also be the distance between the latch on the bike to the rear bar. It is unlikely that both seats are incorrect.
Inspect the bike and find out if the latch has been moved or the rear bar (maybe the fender) has moved. Another possibility is the previous owner added an accessary on the bike, under the seat area and it is preventing the seat from going down into the latch.
