There's 2 possibilities which come to my mind here.
As michael769 has noted, the signal strength and quality has to be good for DAB signals (stronger than for corresponding AM or VHF analogue broadcasts), and this is something can can be reduced for a variety of reasons. You didn't tell us if they rely on an external aerial or small indoor (or an even smaller one within the receiver) aerial. Both of these can lead to a loss in reception - external aerials suffer wind damage (cease to point in the correct direction, cable damage, water ingress, failure of mounting bracket);
indorr (or within the radio) aerials will suffer from new obstructions between the transmitter and receiver. From your username I'll guess that your nearest DAB transmitter is Emley Moor near Huddersfield. Has something appeared between it and you? That could be a new building, a power line, some roof insulation or the neighbour's kitchen appliances!).
The second possibility is equipment failure. If you've noticed the same loss on several receivers, then its definitely the signal path to your aerials that's the problem, but digital receivers do develop subtle faults which can degrade their reception.
Finally, are you aware that forum member Justin Smith is in South Yorkshire and has a business dedicated to broadcast aerials and systems? They will be able to come out with a signal strength meter to determine where the fault lies.
I did some work with one of Sheffield's radio stations for a while, and I did find that the hilly terrain created quite a few significant anomalies across the city, and the combination of a hill's attenuation of a signal with a building's reflection of the same but delayed signal could often create difficulties for listeners.