I keep getting a message to contact itunes support when I try to make an in app purchase. What could be causing this issue? My credit card is up to date and in app purchases aren't restricted.

Have you tried contacting iTunes Support and asking them ? We are fellow users here on these forums, it could be due to a number of reasons, we won't know which one.

http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

I have this same problem and I suspect it has something to do with a limit/liock that they issue on their side.

I was doing multiple purchases over a short period of time, before it suddenly stopped working.

I contacted iTunes support and they said they "made the changes" for it to work again.

It was fine until I tried to make multiple purchases over a short time again, so now I am back to square one.