If the trees are in line of sight of the sat, then yes they will block the signal. Please use
http://www.dishpointer.com/ and locate the dish. Set the satellite to 28.2east. This will tell you if the trees are in line with the same angle of the sat. It will not inidate however if they are tall enougth to be in the line of sight, but its a good place to start. TBH if there dish is on the chimney, then the trees would have to be very all and/or very close for it to be in line of sight.
There could be a number of other reasons why signal has gone bad for them:
1. dish has moved in the wind
2. LNC is on the way out
3. Water int he coax
4. dish ahs become rusty and thus wont reflect the signal well.
5. digibox's IF tuner has broken.