Problem with fuzz pedal

A few months ago I bought a mooer fog pedal (fuzz pedal) for my Spector Rebop. When it arrived I was really looking forward to hear how it sounded so i did, everything was alright so i played around with it like a child, it gave a cool fuzz tone to the sound and made it sound great. So i thought i could use it in the gig i had the day after that without trying it on a rehersal... bad mistake... Turns out that when i play it along other instruments the sound from my bass gets totally muted to the point to which you can barely hear it.
A week later it took it to a rehersal and verified that when i played solo everything was cool, but when other instruments join in the sound gets absolutely muted, the owner of the studio said that his best guess was that since my Spector bass is active and a fuzz effect is quite unstable what is probably happening is that my bass is overloading the input and so when other instruments are playing it becomes apparent that the signal from my amp is cut up or something like that and it becomes muted. I don´t know about the theory of it but I did try his passive bass and it worked alright, his advise was taking it to someone that can put a buffer into the pedal.
Since i know nothing about pedals and I lost the original packaging and cannot give it back ( I know im an idiot ), can someone tell me their thought on the matter, if i should try to put a buffer on it and it would be worth it, or if i should just try to sell it second hand

By the way this is the pedal http://www.thomann.de/es/mooer_fog_bass.htm