DDJ-FLX4 Recordings volume fluctuating

A couple of my recent mixes I have recorded with my DDJ-FLX4 are having some serious fluctuations in volume level which is not noticeable whilst the mix is taking place.

Can I fix the recordings already made to equalise / stabilise the volume level? 

Is there a way to prevent its recurrence or do I have a fault with my equipment.

Thanks.