Dr. Alexiades tells us, “Silicones such as dimethicone, amodimethicone and cyclomethicone are commonly included in sunscreen because they create a barrier on the skin’s top layer to protect against environmental stressors such as wind and water. This increases their overall efficacy but can also cause the formula to clump together on your skin.”
Other common culprits include xanthan gum, which “is used in countless products as a thickener, stabilizer and binder to enhance a product’s texture and feel but can lead to pilling when included in higher concentrations” and carbomer, “which needs to be neutralized properly” or else it can cause the sunscreen to ball up.
TL;DR: If your sunscreen is always flaking or pilling, check the label for ingredients that might be causing this to happen like dimethicone, amodimethicone, cyclomethicone, xanthan gum and carbomer. While this isn’t an exhaustive list, it’s a start.