limitup, you can confirm this by following the 'Edit Campaign Settings' link towards the upper left of the Campaign page.
Click the 'Recommended Budget' link in item number two. If it gives you a number bigger than your actual budget, then your ad won't show all the time.
Handy Tip: If the recommended budget is larger, you can estimate how often you ad is showing by looking at the ratio of your actual budget to the recommended budget. For example: If your actual budget is $10.00 and you Recommended Budget is $15.00, then your ads will be showing 2/3rds of the 24 hour day - or 16 hours out of the 24.
AWA
Another thing to consider, the daily budget is based off of the average day. So if on day 1 it would take $75 to get all the traffic, and on day 2, it would take $25, then the average is $50, however, on day 1, you'd only have gotten %66 of the traffic ($50 of $75) as G stops showing your ads when you hit the daily mark.
We generally add 20% to the G recomended budget (and for some small terms upto 500%), to catch all the traffic.