"Recently I decided to take one of my condenser mics out with me on a live gig to see what it would sound like, but I couldn't get it to work. My live mixer has phantom power and I did have it on. I later took a different mic out and it worked, but the first mic never did. Is there some compatibility issue with phantom power?"

Your problem most likely relates to the supply voltage. The standard voltage for phantom power is 48 volts. However, many mics will operate on much less voltage, and there are a few that require more than that (though they usually have dedicated power supplies). Not all mixers - especially inexpensive ones - put out the full 48 volts of phantom power, which normally isn't a huge problem since many mics can operate on voltages as low as 9 volts. For a mixer to produce the 48 volts a dedicated 48 volt tap off of the power supply transformer is required. There are many inexpensive transformers that provide + & - 15 or 17 volts, but no 48 volt tap. This is a way manufacturers can save a few bucks and bring out products more affordably. Unfortunately if you have a mic that requires a full 48 volts you are going to have to invest in a dedicated phantom power supply unit. There are several makes and models available. Talk to your Sweetwater Sales Engineer for details.