Leading 2-1, Town eventually fell to a 3-2 defeat in a game Sampson felt they 'dominated for long periods'.
"We didn't show enough professionalism to see the game out," he told the official club website after a second defeat in a row for his side.
Sampson's aim of a top-half finish remains intact and the Cobblers will seek to take a step towards that when they travel to Hereford United on Tuesday.