Dogs have little bumps all over

I have two mini schanuzers and they both have bumps from their necks down to their tails and even some on their bellies...their tiny little bumps and when i brush them it looks like dandruff.. i thought it was fleas so i put them on Frontline like 2 weeks ago and the bumps are about the same..i dont know if their allergic to their food but theyve been eating it since ive had them about 3 years(Merrick Grammys Chicken Pot Pies) and their only treats are dehydrated chicken or beef..I also ordered natural nematodes for the yard to kill the fleas in case if thats what they have..Early this spring Ive been walking them up the street usually I just have them in the backyard..So I dont know maybe if thats how they got the fleas..Im just really worried about them...Their everything to me! Please Help!