Some locksmiths can program the BCM some can't, just call around. You don't program the key, you program the Body Control Module to accept the key. There's a lot of components to the system, BCM, antenna, wiring, chip transponder in the key, etc. I would buy a second key, have them BOTH paired to the BCM and see if the problem persists, you likely have one key failing but if they BOTH stop working then you have an antenna or BCM problem. Either way you now have a spare key for the car since it sounds like you only have a single key.

 

Well had laser cut key. Have to have the proxy key almost stuck on the switch to get it to turn it over. But it does it and better.
But now my battery is dead. Putting out 13v. Think that could effect the Nissan signal thing?