Hi Andyboy,
Immediate same station exit doesn’t work at validators, only gates. With validators you need to wait until 2 minutes has passed at which point the cheapest single fare from that station will be charged. However, if the station is within the zones covered by your travelcard then there is no penalty for not touching out, you can just leave the touch in there. Finally, if the reason for not travelling was disruption or no service then the helpdesk will refund the entry charge if one had been made.