99.99% of the time this is for two reasons. Here are the things to look for:
Your c-client and PHP combination is almost certainly the culprit. For whatever reason, sometimes PHP and your local c-client version just don't get along. Strangely enough, you may not receive any errors at all while compiling PHP but when using IMP you will see strange behavior. If this is the case, the first step is simply to upgrade your PHP and/or version of c-client and recompile. Sometimes, just re-compiling PHP without changing anything will cause it to work. Don't ask us what's up with that.
Another issue occurs when a malformed message in a folder crashes c-client. Generally this occurs when a malformed MIME header in a message (much of the time from a spam message) causes a buffer overflow error (or something similar) in c-client. There is nothing IMP can do about this, and it's probably a really good idea to upgrade anyway due to the security concerns alone.
This is amost certainly *not* an IMP problem. Before you go and run to our bug reporting system or mailing lists and complain that we aren't displaying messages correctly, please look at the following:
Obviously, there is a chance it may be a bug but it usually isn't.