|There is the possibility that the line you are having problems with has been in situ longer than the other and has been broken and repaired a numbers of times. Cable joins don't matter much as far as telephones are concerned but too many of them can have a detrimental effect when it comes to DSL|
Most DSL routers have the facility to look at the technical detail of the line. Something like this:
Noise margin (Down/Up) 4 dB / 5.8 dB
Line attenuation (Down/Up) 26.0 dB / 15.1 dB
Take some reading and compare them with each line using the same router for both..
With Noise Margin or sometimes SNR (Signal to Noise Ratio) the higher the better.
With Line Attenuation the lower the better.
If there is any radical difference you can present that to the engineer.
This will give you some information on what to look for: