Is your x ray grid focused?
Convergent X-ray grid is the plane of each absorption bar, which converges into a straight line x ray grid at a specified convergence distance.
If the focus of the X-ray tube is on the virtual line, the transmission of the primary radiation is the greatest because the absorption bars project the least amount onto the image receiving area.
Any deviation of the focus from the virtual line at the convergent distance will result in an increase in the projection of the absorption bars onto the image receiving region and thus decrease the primary radiation transmission.
This deviation may be perpendicular and/or parallel to the plane of the anti-scattering filter grid.