It's hard to say without being able to see what's actually happened, but my first thoughts are:
1: Insufficient mixing of paint and thinner. After adding the thinner to the paint, stir very well before putting the thinned mixture into your paint jar/cup
2: Moisture contamination.
(A): Do you have a moisture trap on your compressor? If not, you could have moisture in your air line which is being deposited on your model.
(B): Condensation on the wet paint during or after application. It's pretty cold in South Australia at the moment.
(i) Condensation may form in the paint stream due to temperature drop caused by evaporation of thinner from the paint between the airbrush and the model.
(ii) Are you painting outside and then taking the still wet/semi wet painted parts into a warm house?
In either situation, you could be getting localised condensation on those parts, which may leave marks like you're describing. (Typically a whitish haze, sometimes specks, rings)
Note that (A) and (B) are not directly related and are separate issues.