"LOCK MODE TO WAIT" seemingly ignored

Hi. I'm programming in ESQL/C on Solaris for Informix 7.12. After my "CONNECT" statement, I issue the statement "SET LOCK MODE TO WAIT", and then my app does lots of "INSERT" operations. No problem when I run the app alone. But when I try to run multiple copies of my app concurrently, once in a while the "INSERT" fails, returning SQLSTATE "IX000", SQLCODE -271 within a second of real time of when the "INSERT" was initiated. The exception messages fetched are "Could not insert new row into the table" and "ISAM error: key value locked". I thought that if I set the lock mode to "WAIT" in my app, I would never have a statement fail due to concurrent accesses/locks. Things might hang a long time, but would eventually gain access to the table and execute OK. If I try supplying an explicit time ("SET LOCK MODE TO WAIT 10", where the configuration file specify DEADLOCK_TIMEOUT as 60) the "INSERT" statements that fail still fail with a second of their initiation. Where am I going wrong? Thanks. MLS