I've realized that all of my main characters are flat and boring. What now?

I've gone a decent way into 3 projects so far and I've noticed that my main characters are always so uninteresting. The curious thing is that this only seems to happen with the main character. I'm extremely happy with how I characterize the rest of the cast.

I guess I'm wondering if anyone else has encountered this challenge specifically with MC's. Does this phenomenon have a name? What questions do I need to be answering in order to understand what's going on with my character writing and bring my MC's up to par with the rest of my characters?

**Edit:** I was not expecting such a response! Thanks everyone for your feedback. I may not get to respond to everyone, but I promise that I'm reading all I can.