Sometimes you can find a process hanging because of some kind of system problem. For example, if the processes was doing some disk I/O operation, it might get stuck in uninterruptible sleep ('D' disk wait in ps report, 'U' in top), which indicates either that something is broken in your kernel or that you're using NFS. Also, usually you find that you cannot kill -9 this process.

Another process that cannot be killed with kill -9 is a zombie process ('Z' disk wait in ps report, <defunc> in top), in which case the process is already dead and Apache didn't wait on it properly (of course, it can be some other process not related to Apache).

In the case of disk wait, you can actually get the wait channel from ps and look it up in your kernel symbol table to find out what resource it was waiting on. This might point the way to what component of the system is misbehaving, if the problem occurs frequently.