as above we need the model of the etu to offer anything more concrete.
for a setup that uses optical sensors (titan/aster/perun optical) they can be sensitive to grease etc and you have to be more sparing in that regard than an external unit that doesn't care if the box is drowned in grease.
for setups that use mechanical switches, then the area to look at is the cutoff lever and wether or not it's got the travel to activate the switch, had this problem with an ascu unit many years ago and had to loot my local shop's box of random cutoff levers to find one that had the right travel.