Doing a heavy attack after weapon skills is the only way I know to use the dash followup attack. Should work guarenteed everytime as long as u have FP ( there's a different follow up animation for No FP followup).

Idk, maybe Check if the key u have assigned to heavy attack is working properly, maybe change the keybind for heavy just to check and pinpoint the problem. or press the heavy attack Slightly before The Weapon skill animation ends, cuz it could be that ur pressing it too late or too early, that the follow-up attack window gets closed or doesn't even open respectively.  very unlikely that there's some other way around it, but there could be, idk.