SM not enabling NAT

About 1 in 15 SMs we try to program for deployment have NAT disable d and does not let us enabe it. The option is greyed out and will not let us click the radio button. I've searched around for solutions and can't find anything. Anyone have any ideas?

Hi, What version are you running on? Are these SM’s are managed by CNS server? Is this on one SM or multiple SM’s?

I'll try to provide as much information as I can on this. There are several SMs of different models/versions we have run into with this. To give a more specific situation, the one I was working on today is a canopy 5760SM. Running software CANOPY 10.5 SM-DES. This happens when plugged into a laptop just to configure the settings before deployment, so it would not be on the CNS. 

Update: I did find a work around. Apparently resetting the SM to factory defaults  enables you to configure the NAT. That's not too terrible of a work around so we'll continue to do that, but if anyone could shed some light on why this option beomes unavailable, the more knowledge the better.