I think it's better to feed "meals" and not have food available all the time. If they don't eat as scheduled you can tell if they are not feeling well. Also eating at the same time means pooping at the same time everyday as well.
Also it lets them know that you control the food and not them.
But I had so many food "fights" with Uni before. i tried so many different kinds, Royal Canin, Eagle Pack, Science diet (the natural kind), some hollistic stuff, mixing with canned, warming the in the microwave, spoon feeding (yes
) but when I changed her to wellness she loves it! So maybe your pup doesn't like the brand of food you're feeding?