After replacment of 2 & 4 glow plug, the engine check light is still on

Hi,
In jetta tdi pd 2004 the codes have said, I need to replace glow plugs in 2 and 4 cylinder.

i replaced them and erased the codes. The milf lite went off. After few yards of driving, the milf light came on again. It happened to me twice already. The firs time the milf light was off longer [~100 miles?].

any suggestions?

sincerely,

aj