M&W 4590 plugging pickup

I have anM&W 4590 and when the chamber gets about half full it will not pickup any more. I have rolled as slow as my tractor will go and payed with the RPM’s and the pickup height. I was able to make one test bale with it after buying it but it was short thin grass that was really dry and it sucked it up no problem. All I can think of now is to remove some wheels from my rake and make a much smaller windrow. I may be overthinking this or just plain forgetting something, it is my first attempt at making hay by myself.

M&W 4590 plugging pickup