Did you upgrade your ariel to a digital when the change over happened?
There's no difference to the actual aerial regardless of the type of signal, despite what the marketing folk might claim. There has been a continuous process of re-tuning transmitters to clear spectrum since the system went digital, however, and this is still ongoing, so, if the aerial is old, it may well no longer be of the correct spec. for the transmitter you are receiving.
Another thing to check is your downlead. These can get damp inside and become quite lossy.
However, if the TV reports a good signal strength but poor quality, chances are that it's multipath or interference that's the problem. Make sure the reflector on the rear of the antenna is still intact so it rejects local reflections and that it's accurately aligned with the local transmitter. If you have had to put attenuators in the path before, try removing them, and, of course, if the signal is this strong, any booster amps will be doing more harm than good.
Get rid of any splitters / combiners, etc. and simplify to an aerial attached directly to one TV if you can. See how that behaves first.
To answer your question, attenuators are just a couple of resistors in a tube, so they are normally reliable but they add a couple of extra connections and any connector in the system has the potential to become dodgy!