Do you feed them at a regular time every day?
If so just switch to the flake food at meal times.
If they don't eat it then take it out after a few minutes and don't feed them that day.
Try again at the same time the next day, if they don't eat it then perhaps don't feed them that day too.
I'm pretty sure they'll be ok if they miss a couple of feeds...I would definitely feed them the frozen food on day 3 if the STILL wont take flake food though.
If you don't feed them at a regular time, then start. I've had a few fish that refused certain foods for awhile. I tapped the tank lid lightly at feeding time before putting food in.
Eventually when it came roughly to feeding time a couple of taps on the lid and they were straight up to the top whether I put food in or not.
In the end I just switched the food and if they're hungry they'll eat it.
Plus if you're feeding bloodworm and brineshrip only then they'll probably a bit fat/full up. Giving them the option to reject any other foods.
You're probably right about them needing a slightly more balanced diet. I don't know EXACTLY how important it is though.
The above methods worked for me when I had fussy fish though so hopefully would work for you too. Good luck
