A phono cartridge output voltage is very low. The music cut into the grooves of a record is lacking in the bass and boosted in the treble. A phono pre-amp boosts the bass and cuts the treble and provides a lot of gain.I took a look at the mixer you use and it has 3 phono inputs, that is three channels of phono pre-amplifiers.
http://www.soundandlightcity.co.za/DJ%20Mixers.htm
Your turntables should only work when pluged into the "phono" inputs on the mixer.
If one of your turntables worked and now it doesn't, I would think there is a connection problem with that turntable.
Sorry I couldn't help more.
Tre'