I'm with kenc. Without a proper footer, and a way to relieve the pressure against the wall, it will eventually move. Also, did you just make a straight wall, or did you curve the wall back into solid dirt on each end? I realize than retaining wall blocks are stepped and overlapped, but having "wings" to tie the wall back into solid dirt would probably help also.
The type of fill used behind the wall will also play a role, (and this ties in with the drainage issue kenc mentioned)..ideally, you would want your fill to be at least level, or preferably slightly taller than the top of the wall. This will encourage water to run over and around the wall, rather than soak down in behind it, creating hydraulic pressure. Did you "cap" the pond muck with topsoil and reseed?
