I had this problem as well. I sent a copy of my previous W-2, 4 recent pay stubs, and a copy of the CBA (I pointed out the relevant sections to define how we get paid, and focused on the minimum so they couldn't accuse me of inflating my earnings; I also pointed out that our life insurance policy was based on 960 hours of pay per year).
This solved the problem. IOW, drown them in information!