User blog comment:Otakon7/Otakon7's Buddyfight Simulator/@comment-27352927-20170210070236/@comment-2234004-20170210104351

Sometimes the sync between players lags a bit which causes an action to not register, it usually fixes itself if you repeat the action (so in this case, if he returned the card and played it again, it should show up)

If not, opening a browser console (F12 in FF and Chrome) and selectin the console tab, then doing the action should give you a notice if an error happened.