Just had a quick look at the system via the link you provided.
http://www.aquasafecanada.com/maximu...-instructions/
OMG, if that's your system it's way complicated, what with the pressure tank & "remineralization" stage! So many things that can go south in something that complex. I assume you have one or multiple valves that allow you to bypass the pressure tank when you're making DI water for your fish tank? Page 12 of the instructions you linked to mention that a change in the factory set 7 psi of the pressure tank air bladder can cause the Auto Shut Off Valve (ASV) to malfunction. Something else to look at. Again, personally, quite a while back I completely removed the ASV from my system. Once I started thinking about how the system worked and the fact that I don't use a pressure tank nor a float valve in my water reservoir, the ASV was obviously redundant, so the choice to take it out was a no brainer. Besides, when my system was new & I tried to use a float valve, the waste water never did shut off completely, couldn't get the ASV to function as advertised no how. Now that it's gone, I also get better pressure at the membrane. Of course you need the ASV in there as long as the pressure tank is there as well. Anyhow, still might be an idea to troubleshoot your system by completely isolating the pressure tank circuit & without the ASV if that's not too much of a hassle.