Could be cleaning the cooking grate too much, so that it's not seasoning correctly, or not cleaning the grate enough so that burnt food is sticking to fresh food.
Could be too much sugar (including tomato and fruit juices) in marinade / sauce, which is like glue
Cloud be moving the food too soon or too much. While searing food will stick and then release, but then start to burn and stick again. Let it ride, remember Weber's motto - Lookin aint cookin
My Weber Q1200 has cast iron grates without any coating. They stuck for the first few cooks, but are nicely seasoned now and only stick if I go nuts with sugary toppings.
