Upstream rate only half of achievable speed

[#1]

Up to the end of March my upstream speed was a steady-ish 15 Mbps.  Since then it fluctuates between 10 and12 Mbps.  Using the BT Wholesale Broadband Performance Test shows 10.13 Mbps with an achievable speed of 20 Mbps.  My download speed is between 35 and 38 Mbps.  The IP profile is 38.04.  Is there a problem with my upload speed or can it return to nearer 20 Mbps?
I look forward to hearing from you.
David

[#2]

Your upload sync has dropped to around 12mbps. It's not down to DLM and our testing isn't finding any faults on the line.
We'd struggle to raise this to our suppliers I'm afraid as it's not a big drop.

[#3]

I would call half the speed a big drop! 

[#4]

It hasn't dropped to half the speed?
It was at around 15Mbps, now it's synced at 12Mbps.
The BT Speedtester will show the upstream achievable speed as 20Mbps because that's the wholesale service it's provided on.
The current speed is still above the upstream estimates for the line so as Matthew says we won't be able to raise this as a fault.

[#5]

@djtoastie
All FTTC lines show the upstream IP Profile = the maximum speed for the product.

[#6]

No explanation for the drop in speed then.

[#7]

It could be line interference or crosstalk both of which can cause the drop your seeing I have gone from 65Mb down 10Mb up to 55Mb down and 5Mb up due to interference and or cross talk or so i'm told by plusnet support etc,  it also could be the start of a developing fault, there isn't anything you can do or at least there is nothing Plusnet or BTW will do until the speeds drop below your estimated range.

[#8]

Thank you Terranova667 for your input.  It's a shame plusnet can't offer an explanation, rather than three letter acronyms.

[#9]

Quote from: Terranova667

it also could be the start of a developing fault, there isn't anything you can do or at least there is nothing Plusnet or BTW will do until the speeds drop below your estimated range.

Incorrect - if you fail the KBD checks, Plusnet can request an engineer to fix the detected fault.

[#10]

This is sort of implied in the answers already, but to be more precise .....
DLM does not apply a profile on upstream,it is defined by your package; in your case 20Mbps.
BTW test result uses the word 'achievable' to relate to the DLM limit, which is at best confusing or misleading, and is really just plain incorrect.
So, your line is performing reasonably, even if not quite as well as before.
The drop in speed may be because of crosstalk with new users on the cabinet, some other slight degradation on the line, a noisy power supply or motor somewhere, or (if you are testing wireless) because of extra wireless conflicts.
Your main and justified gripe is against the confusing wording on the BTW speedtest, and against the fact that it is not as easy as it might be to get at other stats that show how the line might perform and what is really 'achievable'.