I'm presuming the pistons are grooved and the circlips are internal style? Do the problem circlips have an ear with a hole? Are they fracturing through the hole, or through the circlip near to the ear?
IMO, I would say the circlips are of inadequate strength, and thrust is fracturing them. There are some thrust calculation guides in the linked BSC document below.
I would be looking at the possibility of machining the gudgeon/wrist pin grooves wider, to accommodate thicker, and therefore, more robust circlips.
I would also be investigating the availability of circlips that don't utilise ears with holes, if this is the style currently being used (i.e., plain circlips, that are commonly used on driveshaft universal joints).
http://www.bsc.com.au/sites/default/...s/Circlips.pdf