Does the "+" key work in other apps on your computer?
If it does then try verifying the install. If not then a new keyboard will be best option.
OldFlak....
System Specs
i7-9700K 3.60GHz. ASUS NVidia GeForce GTX 1060 6GB. 32GB Themaltake ToughRam Z-ONE 3600.
Main Screen: HP 27" @1920x1080 - Screens 2\3: Acer 24" @ 1920 x 1080
Windows 10 Pro 64-bit Insider
aka Reliquia