So pulling heads after head gasket failure I see circular indentations on piston

- have it on #1, #3, #4 and number #6 pistons. Slight mark on #2. Was not there a year ago

Quite shiny as well - thru the carbon, so must have happened recently. Any suggestions as to cause and possible remedies?
Have checked the cam timing. Was my first check. No problems on valves. Seated nicely away from piston.
Sticky valves - how to check? Same with valve springs.
Thanx in advance.