ECM drones are chance based and the random number generator seems a bit "streaky" sometimes (is that a word?) so sometimes you just won't get lucky with them procing.
Other times they'll seem to work all the time (usually when a hostile gang has them on you).
A lot of this is probably just perception (you remember it more when they don't work for a whole fight).
There are lots of things that affect how likely they are to work (as detailed above) but don't forget that they are random and not guaranteed to work.
They're never going to be as good as a dedicated platform with multiple bonuses.
For what they are though (some "bonus" e-war on boats with no bonuses) they can be great. A lot of players think they're OP rather than underpowered.