The difference in attenuation your getting is very odd, one of the following could be a cause.
Corroded RF2 filter.
this is something fitted bfore your master socket. Intended to reduce any pick up of backround radio on normal voice. (overhearing radio 5live during a conversation on the phone)
If you have an rf2 then you shouldn;t get any dsl sync at all, unless your very close to the exchange or the rf2 is corroded & letting some dsl slip through. If you have an rf2 then it's a case of caontacting your isp to get a engineer out to replace it with an rf3 which was designed after dsl was invented and lets the dsl go through unimpeded.
How to find out if you have one, this will be a small white box about the size of a matchbox before the master socket maked bt. 19 times out of 20 such a box is purely a connection box. If you take off the lid (carefully this is a bt owned item, disturb anything then it is a chargable visit to fix). Unless a cable is obscuring it then inside you'll possibly see rf2 printed. and an rf2 has 2 small black cylenders about 4-5mm dia 6-8mm high. (an rf3 would have an 8mm square block with a visible copper coil within).
Ring looped internal wiring.
Rather than your internal wiring from the master socket just going off to your extention sockets. this cable carrys on retuning back to the master socket. so your dsl signal splits off & meets itself comming back the other way causeing "Reflections" so causing alsorts of issues with the dsl signal quality. Can you disconnect a cable from the master socket with all the extention sockets still working. this usually means a weekend with a screwdriver & sore knees.
Is your property fed by an overhead cable, follow the cable from the master socket to the outside of the property. is part all of the remaining cable back to the pole a grey cable which looks like 2 wires bonded together (don't mess about with this cable, mainly in case you've got hold of a mains voltage cable instead of your telephony)
