Google doesn't have unlimited inventory. Only a certain number of people actually perform the searches that you are bidding on every day. Other people may also be bidding more than you and you ad is not showing up as often or as high as you would like. If you want more volume you can:
- Bid on more keywords
- Increase your bids
You indicate that you gave this keyword a "test". If you ran a test for a short period of time, that test may not be a great indicator of future performance. When you first put keywords into the system, Google doesn't have enough data to price them for you properly. You may get more or less traffic during this time period than you will when you bid longer term.
There may also be a problem with your "quality score". Google crawls your landing pages and tests showing your ads. Based on these tests, it assigns your ad a score. Much of this score is based on the click through rate (CTR) during these tests. The better your quality score, the more you ad shows up even when you have low bids.
You say that you "moved it over to it's own campaign". When you re-arrange your account, quality score may have to be re-computed. If an ad is working in a specific campaign, it likely has a good quality score there and I would recommend that it not be moved.