Mower pushes over high grass not cutting, requires second pass

Hi,
If my grass or weeds are about 18" high the mower fails to cut, but just bends them over and does a little cutting...it requires a second pass in the opposite direction to cut. I was wondering if anyone has cut open the front of the mower deck allowing the blades to be more open to cut the high weeds? Please no opinions just real experience.

I regularly cut down weeks 5' tall with my Greenfields 16/32 Evolution without any problems.
The 1966 Rover Rancher II has no problems either
The landlords 2000 series Cub Cadets will happily chop down 4' tall rye grass, lantana, Blackberries , tussock grass & Pattersons curse.
None of them have any modification to the deck