First, are you using the proper thinner (also known as reducer) for the application? Different thinners do different things to paint. Because all paint thinners are not created equally, always use the thinner specified by the paint manufacturer for the specific application type.
Generally, spray thinners allow paint to atomize properly, and evaporate much quicker. Brushing/rolling thinners give extended flow and self-leveling properties to help with working times, accommodating a wider temperature range.
It's important to note that thinning doesn't extend square footage for a paint, it only makes application easier.
Brushing/rolling: Most paint manufacturers use good judgment and will tell you if a paint needs to be thinned under the most common application conditions. Typically, you'll see a range from no thinner needed to 5%, under most conditions. As the temperature increases, you may need to add 5-15% reducer to make the paint flow, while trying to maintain the wet edge. If you notice that the paint is dragging on the brush or roller, you may need a little more thinner, and, if it's not giving good hide, with dripping and sagging, it may have a little too much thinner. You should never feel uncomfortable contacting the paint manufacturer to get their thinning recommendations—they want to help you achieve the best paint job possible.
Spray application: Spray application is sometimes more of a science than brushing and rolling. Spraying properly is a fine balance of having enough spraying thinner, the proper tip size, and adequate pressure to get proper flow. Many paint manufacturers will give specs for the spray application type, thinner, tip size, and pressure. It's not uncommon to see a recommendation of 15-25% spray reducer, sometimes more. If you're unfamiliar with spraying a certain paint or how to spray in general, always set up your equipment, get the proper spray tip and pressure, then try spraying with the minimum recommended amount of thinner mixed into the paint. If the fan is not looking proper or atomizing as it should, it may sputter. To correct the problem, do not add a lot of reducer at a time. You also want to avoid adding too much reducer, which could lead to thin, drippy, or sagging paint.