Problem: A REXX program works sometimes and sometimes not.
Hint: If the program uses file I/O, check if you're closing all opened files. The number of file handles is limited in an OS/2 session. (see MaxFH on how to increase the number of file handles; see lsof for getting a list of all open files)
Please note that the entry FILES=nn in the file CONFIG.SYS is only valid for DOS sessions!