Diagnose that sucker, and then you can find the problem. Sounds like you're using new tubes instead of patching the old one (there's NO reason not to patch, but leave that for another day), and if you're not seeing the hole every time, that's contributing to your problem.
It's important to find out whether you're having the same puncture over and over, which would indicate a problem in the tire or the rim, or a new problem every time. Next time you get a flat, pump the deflated tube up and put it in a sink full of water to see where the bubbles come up. Mark that spot and patch the hole, then look at the rim and the tire right where you got the hole. If it's on the tire side of the tube, you probably have a little piece of broken-off thorn or something stuck in the rubber. If it's on the RIM side, look for a break in the tape or the end of a spoke protruding too far. A really tiny thing will do it--there's a lot of pressure in there.
One more tip: When you mount a tire, mount it so the label is right at the valve hole in the rim. Then when you have a flat, you find the hole in the tube and use the valve and the tire label as landmarks to see where in the tire you should look for the thorn or whatever. Saves a lot of time over the years.