It's a common issue (actually to common and should be bug reported every now and then - looks very untidy).
It's occured with weapon- and/or stance shifting for me. It sometimes cures by just that aswell; relog works aswell.
Even if you loose the icon of Flying kick, pressing G (defaulted at that anyhow) will make you use our finisher, pressing G again will trigger the followup finisher once you get it at 22.
It's a bit tricky if you want to use Thousand Fists or the AoE Kick though, but you're not missing out on anything to be fair.