New add lasts 1 day then it is not shown

When i place a new ppc add in google, it is shown for 24 hours, get imps and gets clicks then on day 2 it is not shown again
this has happened 3 times.
Any idea why this is?
1st time i got a ctr of 12% click through so the ad got clikcs ie.e it appeadled to its taregt market.
The 2nd time it got to 7% ctr. The after 24 hurt is was not shown again