I've caught that response a few times.
In each case I had to;
Verify that the nrpe.cfg file was correct - check the syslogs and check for messages about an error in nrpe.cfg.
On Linux servers the nrpe.cfg goes in /usr/local/nagios/etc. On Solaris servers I had to put a sym link in /usr/local/nagios.
Since your script is working on other servers ok try to find the commonality of the servers it's not working on - OS, rev level, patch level, etc.