I do not know anything about Prosweet, but I do know this:
1- bees will ignore feed whenever there is a natural source (foraging) available, even with good quality of the feed.
2- bees will be hesitant and/or will not take feed at all when it is cold (less than 7 degC approx), because they cannot process it properly.
3- bees will take feed when they cannot fly; raining / windy
4- bees will take feed when the landscape is void of forage
5- bees will take feed that to me sometimes looks rather nasty but is good in their opinion. When open fed.
6- bees will ignore feed if they do not have a healthy active queen. Often the surest signal of queenless hive or a virgin queen is the hive that is not taking down feed while all the other hives are.
7- bees will ignore feed if they are sick or overwhelmed by pests / disease / virus
If the bees are not taking the feed when conditions 3 or 4 exist, and none of the other conditions exist; Just dump it out - at that point the beekeeper can be sure the feed is foul. Especially if he/she knows it is old stock.
Why are you buying buckets of some fancy marketing name syrup? Mix it yourself from ingredients (white cane sugar). That way you know it is fresh and you know the quality / consistency of the feed.