Ive been getting into fishing with plastics a bit and ive found that for Flathead its best to cast wherever youre targetting (dropoff, weed bed edge etc...) let the plastic sink to the bottom free spooling, lock up the bail when its on the bottom and let it sit for about 10 seconds. give it two nudges then let it sink then wind up the slack then a slow rise and a nudge, let it sink then repeat.
i also find that they will take it on the drop after a rise or nudge.
for bream i use a bit more of a irregular retrieve, use using alot lighter jig head, cast to structure etc, let it drop for a metre or so and use a quick retrieve, walking the dog i think its called and pause the lue evey couple of metres, again the pause or drop is usually whjere the fish will smash the plastic.
I think the retrieve technique you use is all about getting their attention and getting them curious and excited. bream tend to be a bit more aggressive but and like a faster retrieve with a few pauses and flatties will see the sand get stirred by the plastic each time it drops after a rise ir nudge and will just eat it because it has gotten their attention when the sand has been disturbed.