You didn't say what brand of acrylic you are using, and it does make a difference in specific diagnosis of your problem.
Acrylics dry more rapidly than enamels, although they attain full cure more slowly. Basically most of what is happening to you is that the paint is partially drying before it reaches the model surface. Acrylics require slightly different techniques than enamels for application. Since you apparently are an experienced airbrusher, you know that a retarder will help. But you should also experiment with different pressures, distances, and reduction ratios to find what works best for you.
Be aware that unlike enamels or lacquers, there is no universal reducing solvent for acrylics. For spraying, it is hard to go wrong when using the paint manufacturer's proprietary thinner, some of which already include a retarder (e.g., Tamiya). However, the stuff is outrageously expensive. Some acrylics do well when reduced with deionized or distilled water (e.g., Polly Scale), others require 90% isopropyl alcohol (e.g., Tamiya, Gunze Sangyo). Reducing ratios also vary from brand to brand (hence my initial statement) and sometimes even within a brand. Using the wrong reducing solvent can also cause the type of problem you report (and many others that you really don't want to learn about the hard way!)
Oh, and in case you've forgotten (I did.) mix the retarder with the reducing solvent first, then add it to the paint.