lack of concentration during a set

Oh YES!

I have had great sessions turn to dust and very bad sessions end like there was no tomorrow.

Not necessarily lack of concentration. But DJ-ing is a flow thing. In the flow you do it almost automatically, but if you lose the flow, unless you have packs and packs of experience, it’s very easy to miss one or two matches, then start to feel bad about them and mess up even more. Downward spiral it’s called.

Don’t worry about it. Just keep slugging and practicing away.