Plotter Range Problem!

[Caer Urfa]

Ihave a Northstar 550 chart Plotter, (identical to the Navman 5" version) and is also connected to Navman 7100 VHF Radio and all work fine.
When I set the 'zoom in range' to it's 'lowest setting 0.4 miles' it keeps reverting back to the 1 mile range.
What am I doing wrong? or what button needs unclicking or clicking?

 

[alan]

Maybe that chart )or portion of) that is in use does not go down to that detail??

 

[Caer Urfa]

Tried that Alan, same in any area.
Had same C-Map for local area before on last boat with same make plotter and worked fine.