I was playing sealed and after a while I could no longer use the "add a counter ability" on my possum druid Tender WildGuide - it wasn't holding priority and couldn't react to instants or end of turn phases and I wound up losing the game because I couldn't keep pumping the possum

earlier in the game I think I was able to add the counters as expected because there were other things (like food or land ability) which held priority.