After a discussion with my Head of Training I have come across another potential C-152 spin problem. Apparently with two very tall occupants (i.e. seats right back) and at certain fuel states, although the calculations will show that the aircraft is in the C of G limits to spin the spin can flatten considerably due to aft C of G, and be very difficult to recover. I will now always be very careful that C of G is well forward of the aft limit before spinning a 152 again.