A gate (aka noise gate) is a fancy automatic on/off switch. It turns on when the signal coming in is louder than a certain level that you can set. And it turns off when it’s below that level. For example a guitar amp often generates a bunch of noise even when not being played, which generally you don’t want to record. A gate will only let the signal through when the guitar is actually being played, thus keeping out the unwanted noise. But if it is not setup correctly it could also cut out some of what the guitar is playing. Here’s a detailed description Noise gate - Wikipedia
Like Grim said it would be on an insert. Just turn it off like any insert. Or open it and learn how to set it properly. 
A gate is your most likely culprit (but you should still play with mic placement to get the best sound).