at a guess is it possible there is still activity on the serial input ie a cr/lf after the command.
your code might see a RCIDL condition between these chrs and pop into and straight out of sleep
try a small delay before calling sleepmode a 2chr time delay can't hurt here

It often makes a difference adding the NOP right after SLEEP.
which you have already is necessary, the extra two in use may not be adding much value but won't hurt