The biggest problem with the switch to digital TV is substandard antennas.
In analog days, people coped with snowy pictures - you could enjoy the programming and listen to it but not get a clear picture. Digital is all or nothing, so it either does or doesn't... and if you're on the edge, some days you get the channel, some days you don't - atmospherics atmospherics still play a part with your TV picture. There are some days I don't get all the Raleigh, NC stations (I'm in Greensboro) and this is with a good outdoor antenna improperly mounted (it's not on a mast, just sitting on the front deck, not even aimed at Raleigh) - and other days they all come in like anything.
Also is your antenna an indoor one or an outdoor one? Outdoor is always better than indoor, and for a little more money than the cost of a top-of-the-line amplified indoor antenna you can purchase a good quality outdoor antenna and get better results - without an amplifier; my Winegard HD8800
http://www.solidsignal.com/prod_display.asp?PROD=HD-8800 - $45, I picked mine up locally for $50 (avoiding S&H). Of course I would recommend proper installation...
It can also be the station doing work as well - if they have had to switch to reduced power or an auxillary facility. Usually they try to do this work at off peak hours, but sometimes it's necessary for it to be done in other times. However there's no central place to see which station is doing what maintenance without calling the station directly.
Look at the signal level for each channel on the converter box - the higher the better. I don't know what converter box you have, but the two I have got - both from Wallyworld (so shoot me) the RCA and the Magnavox - have a signal meter - the RCA one is better as it's also audible so you can aim the antenna better for a particular station.
You also have a new problem that affects digital - it's called multipath interference. In analog land this would be known as "ghosting" where you see two images. The digital receiver is also receiving two "images" and has to figure out which one is which... if it can't, it gives up and reports zero signal. Again also atmospherics play a part, but for multipath again, one antenna is better than another for this issue.
Sorry you were having issues with your NBC affiliated station. It'll come back and you'll have some bad reception days as it stands with your current set up.