Breakpoints in SSIS not being hit

I've inherited an SSIS package which is really rather horrible. It is now falling over on a particular task, and I'm trying to put a berakpoint in the script to find out what's happening. However, it's refusing to acknowledge that the breakpoint exists.

The error is happening on a data flow task. If I double-click on that task, it takes me into the data flow page, which comprises two components: one that executes a custom script, and one that writes to a database using an ADO.Net provider. Double-clicking the script component brings up a dialog box that has an Edit Script button, and pressing that opens the script editor.

Within the script editor, I can set a breakpoint on any line of code that I like, and I know that it does run through this section of code. I save the script and exit back to the previous dialog box, where I press the OK button. However, the breakpoints I entered don't show in the list of breakpoints, and when I run the package it ignores the breakpoints and simply falls over at whatever line of code is causing the problem.

So, my question is: what do I have to do to get it to recognise my breakpoint? I've been having a very sweary day because of this, so any help would be greatly appreciated by my coleagues as well as by me.