Try these steps
1 Go to windows settings, click on privacy and security
2 Click on windows security
3 Click the tab App and Browser control
4 click on Reputation-Based protection settings and enable everything there.
*I think this might be the answer to your problem.
I did this and in fact reputation-based protection was off, however I can't turn it on again, after I try to turn it on I get a prompt asking me if I want windows defender make changes, I reply yes, but the reputation-based protection is still off, no matter how many times I try, even after a machine reset it's still off.