Assuming you have the 1.4.x firmware installed, you should be able to turn on some Java debug on the phones which will write Java information (and MIDlet information) into the log file. This might give a clue as to what is happening.
The developer guide gives some information towards the back of the document on how to turn on the debug values and how to download the load files to see what is happening in the phone.
Usually, if there is a problem with the jar/jad file, an error message will be displayed on the status line. If there is no response at all on the screen, that would make me suspect that the JVM is not in place.
-Conrad