Ditto above...have you tried reintializing the remote key? It'd be a good thing to do anyways...because it will cancel the missing remote's ability to lock/unlock remotely.
Below are the instructions I've posted many times before on multiple BMW forums...follow every step as written, don't skip, fudge, combine etc. If the key's battery is charged, then you shouldn't have a problem. And when you had both remotes, the one you have now may have been absent when the "working" remote was reinitialized at one point. Did you ever try reintiializing both remotes during the same session when you had them?
TIP: KL R=ignition position 1
Don't confuse/conflate the 3 features deigned into the remote keys. They're all independent systems that work in conjunction or tandem with each other...but are yet separate systems. Many owners mix EWS, FZV, & DWA functions into one, but a you can see in the info below..there's differences (especially EWS).
