North East Herts MP Oliver Heald has been contacted by constituents concerned about delays and cancellations on the service between Watton-at-Stone and London.
He has immediately contacted Managing Director of GTR, Tom Moran asking for an explanation and improvement.
Commenting Oliver said: "I realise that GTR are investing heavily in the Loop and there can be teething problems with new rolling stock, but the absence of drivers needs explaining. I am told that many trains are being cancelled for this reason. This is unacceptable to my constituents and I will continue to press their case."