Maybe the OP doesn’t have administrator privileges?
On my work computer the OS allowed me to install/activate the Windows IME, but then prompted me for an administrator password to switch from romaji to hiragana. 
(I’m so happy I learned about Flaming Durtles. My problem solved!
ETA: I may have been too brief. 
To clarify, the app doesn’t solve my IME issue on the desktop. What it does is improve my phone’s WK interface to the point that I prefer it to the desktop version AND together with the installed Japanese language pack on my phone, I can do everything I need to in one place–a portable place too 
)