Hi ratter,
That grass is awful hard for the blade on the back of the box to get under & stay under... once your under it, keeping an eye on staying under it (backing up slightly, then the blade digs a bit deeper as you go forward), or more & more passes will do it... The angle on the blade doesn't look bad...
Can the MF lower arms be screwed out any? I gain a bit more depth & angle on the JD by screwing them out flush with the end of the threaded rod...
Other thing is extra weight will help a lot... my boxblade is a whole different animal with extra weight put on it. My solution was the old free weights (attached).
A little lower arms, and a lot more weight will get you into that grass faster