There are many reasons that waitFor() doesn’t return.
But it usually boils down to the fact that the executed command doesn’t quit.
This, again, can have many reasons.
One common reason is that the process produces some output and you don’t read from the appropriate streams. This means that the process is blocked as soon as the buffer is full and waits for your process to continue reading. Your process in turn waits for the other process to finish (which it won’t because it waits for your process, …). This is a classical deadlock situation.
You need to continually read from the processes input stream to ensure that it doesn’t block.
There’s a nice article that explains all the pitfalls of Runtime.exec() and shows ways around them called “When Runtime.exec() won’t” (yes, the article is from 2000, but the content still applies!)