Need more info Alex.
What needle/nozzle are you using ? What temperature are you at in the room? What distance are you from the object when spraying ?
Most
Tamiya addicts thin 35%. Opening up a new bottle I fill up the the top of the shoulder of the bottle that is it for the whole bottle use. I then put a little sticker on the cap to confirm it has been thinned.
Most of my work is with a 0.35 needle nozzle. At 6" proximity although I do change depending on circumstances.
I choose 20PSI. I would say 15PSI may be part of the problem but then need all facts.
With all of the above never have any problems.
Another factor. If you have been spraying gingerly around a difficult bit spray on spray off etc. I have a stand with thick paper I give a good full burst every now & then to keep things moist with new paint at the nozzle.
It is worth persevering as this I have found to be a very nice paint with a very thin coat. Very forgiving to much paint & most times you are lucky. Over the top & I quickly wipe with kitche roll & spray over immediatly works well. Shame they do not produce more colours.
Laurie