Please check if you are using PMDG 737 NGX version SP1C or SP1D.
If the version is correct, then this is likely caused by a known SimConnect issue in FSX. In this case, please try loading a default aircraft before the NGX.
This can also be the cause for the EXEC light not working.
If you still got this problem, try a new VA interface version using the link bellow:
Download and place it on the folder you are currently running VAInterface from (VAInterface installation folder).
Run it by double clicking on the downloaded file name.