If they're not omniscient, then they drop things. A few prayers out there that they miss, eh?
But, HOW do they know someone's praying to them? What alerts them if they're not omniscient?
An angel taps them on the shoulder?
It's not binary, you know. Not everyone who knows more than I knows everything.
Okay, except my daughter, when she was 13.
My alleged point is that there are possible gradations here. Mary might not be good at geometry or quantum machanics. She's as good as we need her to be at the stuff we need her to be good at.
We have a very expansive view of what it's like to be with God. You can guess how large the vision is when you read Athanasius.