Bad set? 

Remember this...

It happens, to everybody.

It's not the end of the world. It is a learning opportunity.

It's not always the crowd's fault. BUT they can play a part.

It doesn't matter if it was a small crowd. Personal funny stuff goes over even in very intimate surroundings. Don't tune your timing and mind to big laughs, just laughs, that's all you seek.

*Booming laughs from a big crowd is just small individual groups of people laughing all at once. ;)

Too many bad sets in a row is a red flag that signals back to the drawing board. If you're really into it you will happily do that.

Consider seeing through the absurdity and remember the only rule in comedy is that you commit 1000% to having fun with it. No matter what it is you want to express in your comedy, if it's anger, confusion, frustration, angst, whatever it is, inside your head you must be having fun or it's no good for stand up.