Issues on the client like this are typically Java related. Have you tried clearing the Java Cache on the affected workstations?
Thank you so much, it resolved my issue. But I could not understand that how it is related to Java Cache.
The Java Cache is where the temporary .jar files are stored that are used by your Oracle Forms application. There are times when the .jar file can become corrupt or unstable so clearing them will force the application to re-download them to the client.