I have the same problem.
I tries buying gems maybe 5 or 6 times in the last 2 weeks and the game just wont let me do it. It just quickly flashes some red text after I accept the transaction which says "An error has occured, try again later". I tried lowering my firewall heck i even turnt it off completely, cleared the gw2 cache tried with several other credit cards and still nothing. And I highly doubt it that there is some kind of restriction from my bank because i made purchases bigger than cost of 8k gems.
Any1 knows how fast the support team usually responds?