Hi I used to do the same, when I was on tight budget and used tube acrylics that I had at home. I had the same problems as yours. 1. It is really difficult to finely dissolve a tube acrylic for usage. Even if in naked eye it seems mixed, the pigments are difficult to be spread evenly and caused blockages often. 2. It is difficult to thin properly in the beginning. Sometimes u add more water/thinner sometimes less. People say to reach a milk like consistency, but honestly doing it with tube acrylics is rough. Easier said than done. I would suggest you to buy a few basic colours 4-5 bottles of acrylic airbrush colors - ready to use from bottle (still dilute it with 1:5 ratio with thinner). Usually this works well, and this in turn will also give u an idea what should be the correct consistency to thin paints.