Unable to use Forest Seal vstar ability

I was recently in a match where I had the Forest Seal Stone attached to a Miraidon ex, and for some reason it just would not show an option to use that ability at all. The miraidon was on the bench, i checked for global status that would prevent using it but could find nothing. I had not used any other vstar ability in that match, and neither had the opponent. I tried again in another match and it had the same problem; in addition, moving it to the active spot did not change this and the option to use the ability did not show up as a prompt at all.