Correcting them once is not the problem. Keeping them corrected is. But from what it sounded like, you didn’t mean after 30 minutes their corrected forever, but that you taught it once and then reinforced it over time. That’s what it takes.
One of the hardest things, I think, is correcting the long “a” sound before an aspirated consonant.
They can say pay, but not paid. Tray, but not train. Ray but not rake.
As Joesax was saying, it’s due to photactics (phonemic rules) based on their mother language.
When it comes to spelling with KK, I teach them that KK is an alternate spelling. It show them that KK doesn’t really record the speech as it is usually spoken, and tell them to memorize the KK they need for their tests, but to pronounce the words the way I say them. If they get it wrong, I’ll correct them.
(Luckily, I don’t have to deal with any KK issues right now. I so hated doing that before.)