TV converter box. Why do I now not get channels I could get with analog?

Digital TV reception is subject to what's called the "cliff effect". Reception is perfect up to a certain signal level, then falls off a cliff below that level, resulting in bad image blocking and eventually total loss of picture. With analog, the signal just continually degraded until it was unwatchable. If you got very poor reception on an analog channel, you may get nothing at all with digital. The solution is usually a better antenna.