Hi MArk.
You've been using enamel thinner with Tamiya acrylics? No wonder you're having problems. They are fundamentally different and will not mix. Stirred (or shaken) vigorously enough, you may end up with an emulsified mix which may look uniform, but will settle out, given time, or you may end up with a sticky lump of goo which won't mix at all.
For Tamiya acrylics, in order of preference,
- Tamiya X-20A thinner
- Isopropyl alcohol
- denatured alcohol
If using either of the alcohol options, a little acrylic retarder may be beneficial.
Personally I wouldn't recommend water or Windex to thin Tamiya acrylic paints, though they are suitable for cleaning up afterwards.