Custom keyboard shortcuts using ctrl won't map on a Mac

Hi people, I would appreciate some insight here. I recently came back to Mac from Windows and I am trying to customise my PPro keyboard shortcuts. I am trying to map "Trim backward" and "Nudge Clip selection Left 1 frame" to the left arrow using control and It won't map (and the equivalent commands to the right arrow using ctrl). I've tried dragging and typing the shortcuts and it won't work. The weird thing is that "Nudge clip selection Up & Down" works fine on the upper and lower arrows. Does anyone has come across this? It's driving me mad! Thanks a lot.