Once upon a time, Tamiya's acrylics had nearly magical properties.
Straight out of the bottle, they went on incredibly smoothly, levelled themselves out, and with the right brush looked almost like a sprayed finish..... Then they changed the formula...
Since the early 90's, Tamiya's acrylics are notorious for "roll-up" because it starts to "skin over" super-fast. Brushing back over paint that you have laid down - even just a few brush strokes earlier, will drag up the previously laid down paint.
For best results brush-painting with "today's" Tamiya acrylics, you need to add a little thinner and acrylic retarder - don't paint straight out of the bottle.
Transfer some paint to a pallette and then add a little thinner - how much is really a matter of experimentation. When you have the right consistency, squeeze a single drop of acrylic retarder onto your pallette (not into your paint/thinner mix) and when painting, just touch the tip of the brush to your drop of retarder before "loading" your brush.
It may seem fiddly, but the results are worth it.