Duram Paint: discolour and stink in opened tin.

I bought 3 different paints from Duram: 1. Duram Armatex fine textured. 2. Duram Wall & Ceiling - premium matt. 3. Duram waterproof with fibre inside. In al 3 of these paints I experienced the same problem, and a financial lost. It does not matter if the left over paint in the tins are almost full, or less than half full. After a month or so, when I want to use the leftover paint from Duram to carry on with my paint project, the paint has changed colour and has a terrible stink on it, making it completely in-usable again, and had to be dumped. I have send an email to Duram Smart Paints informing them of the problems with their paint and have not received any reply back. To me it is an admission that they know about this problem, but are not willing to admit it. I have never before when I used paints from Plascon or Dulux experienced any discolouration or stink from any of their paints. Next time I will stick to the trusted paint manufacturers like Plascon and Dulux.