Electronics Experts...

Serendipity

I'm doing my lab report right now which is basically on an instrumental amplifier and was wondering if someone understands this better than I do? I know there are a few speaker builders/electronics guys on this forum so thanks in advance for any help.

I built the circuit on my breadboard using standard 741 op amps and don't get a gain on the output whenever I adjust the variable potentiometer, R4. My lab partner couldn't figure out what's wrong, either.

I understand that there should be a specific range that the gain can be adjusted (that's part of the question) but is there any thing else necessary for this circuit to work? Attached is an image of what I am trying to build.

Thanks!