I would say it's a rare lab that's fussy of what it eats although there are a few, and puppies particularly can be like this.

It's interesting what Miss Macie says as I was going to ask if there was any possibility of the bag getting contaminated in any way.

If all is ok, then I would leave it down 5 minutes and if they don't eat it take it away and give nothing till their next meal. They will soon learn that they need to eat it straight away or they will go without.