Ignoring food is completely normal behaviour. If the food is hard and dry they may come back later when it's had time to moisten up a bit to make it easier for them to wrestle chunks of it, I have found pellet foods break down really quickly, the Ebi Dama I'm using is longer lasting and doesn't break apart like the pellets, it frequently gets ignored but does get eaten. Even fresh foods will get ignored, but will eventually get nibbled on.
When you have greater numbers in the tank the easy to find food sources become scarcer and the shrimp can get very hungry, so when you put food in there's a bit of a squabble to get at it, like you've seen in my tanks. If the shrimp run out of easy food sources and you don't feed them they'll eat the wood hard scape!