...How come, even though my daily budget isn't being exceeded, I'm not seeing my ad every time I enter the keywords I'm bidding on?
HughMungus, in my experience over the past couple of years, I'd say that when an ad sometimes shows and sometimes doesn't, then 98% of the time it is a daily budget issue.
And whether or not you have met your budget so far in the day is not really a factor. In reality, there is a much more complex system running in the background, which (to try to simplify it) is always calculating how many times it can show your ad, for each of your keywords, in the 24 hour day. Many factors enter into this calculation, including how many keywords you have, how competitive they are, how they are targeted, how often they are likely to searched on, what their Max. CPC is, and so forth.
Skibum is quite correct in suggesting that you check your 'Recommended Budget' in the 'Edit Campaign Settings' page of your account, as a first step. If the recommended budget is higher than your actual budget, then it's a sure thing that your ad will not appear 24/7. Instead, the system will start and stop the delivery of your ads in an effort to meet, but not exceed, your budget.
Please note that that it is possible for the recommended budget to show as 'OK', and for the ads to still not appear every time. This can happen when there are a lot of keywords in the campaign with a CTR of 4% and above.
Bottom line, if your ads aren't appearing every time you search for them, it is almost certainly a daily budget issue. And the steps to take are to either raise the daily budget, reduce the number of keywords that are supported by that budget, or perhaps to lower the Maximum CPC for those keywords of lesser importance to you.
This topic has been discussed a time or two in this forum, BTW, so a site search would likely net you additional details.
AWA