Unless your radio is faulty, you appear to have a bad noise floor/interference problem.
What band are you listening too? VHF maybe?
Are you using an external antenna, or an internal antenna near a computer or TV?
Holding on a conventional channel, if you turn the squelch right down and then switch to AM, do you hear any pulse or loud static/crackle type noise?
The 'S' meter on the Uniden scanners only operates when the analogue squelch opens.
On digital channels, the analogue squelch still operates and a full scale 'S' meter can be displayed without any digital audio being produced. Or, if the digital channel you are listening to is weak, you can have a full scale 'S' meter reading (caused by the noise/interference) but high digital error rate and poor or garbled digital audio from the system you are trying to monitor.
For the squelch to open and the 'S' meter to register anything, you must have carrier/noise/interference on the channel strong enough to open the analogue squelch.
You could also try the IFX (IF exchange) function too.
The best idea is to manually tune to a frequency on the band you are monitoring, then switch to AM, turn the squelch right down to zero and find out what the noise floor is.
On a handheld scanner, you can walk around the home, holding the scanner near to various appliances. You will be surprised how RF noisy a DVD player or even a non-operating microwave can be.
Lowering the noise floor (broadband noise level), or putting it another way, improving the Signal to Noise ratio, is critical to obtaining low digital error rates and noise free analogue reception.