CLOSE not sending OK when sent from new modem

[MotionMobs]

We have a system that has been using some older modems for years now with no issues. Recently, we switched to some new modems and now everything is working except that when it issues a ftp close our service doesn't send an "OK" back which is breaking our knowing if the request was process successfully. What could be different that would break this and how might we be able to fix it?