I'm in Toronto's East side on the Danforth. I used to be able to get all of the US Buffalo channels and even some Rochester ones. Now, NBC Buffalo 2.1 etc does not come on at all. Most nights, I get CBS Buffalo 4.1 etc and ABC Buffalo 7.1 etc. The 23s and 29s are great. I had my antenna repositioned last summer after I had trouble getting PBS.

Except for Channel 23 and 29, most US station signals have all become weaker in the last 3 years or so.

Can somebody explain why?

In your case Cocoa, when you repositioned your antenna for WNED, you probably lost the line of sight for WGRZ's transmitter in South Wales.  Refer to the REMOTE CENTRAL map [Link: remotecentral.com].

Or, there may have been a tree growing or a newly built structure (apartment/second floor addition to a house) which has interfered in you line of sight.
Atmospheric and Lake conditions due effect signal reception.
You will be able to receive WIVB a lot better beginning April 13.