When one fixture is affected

A single drain is slow or blocked while others behave normally.

What this page cannot establish

noticing this establishes that you noticed it — it does not establish a cause
this page cannot establish that the buried line is unaffected
this page cannot establish that the problem is confined to that fixture
this page cannot establish the cause of the restriction

What a professional will be trying to work out

These are the distinctions a professional makes when they attend. They are questions, not conclusions — and knowing them lets you follow the conversation rather than just receive its result.

whether the fixture has its own branch back to the shared run
whether the fixture has a history of the same behaviour
whether anything entered the fixture that would not normally
whether the behaviour changes when other fixtures are used

What is worth writing down

These details are cheap to record now and impossible to reconstruct later.

which fixture, and on which level
when the behaviour started
anything already used in an attempt to clear it

Evidence that would change the next decision

If you are deciding whether something is worth paying for, these are the things that actually move the decision.

confirmation that other fixtures on the same level behave normally
whether the issue predates or postdates other plumbing work

What would change the answer

confirmation that other fixtures on the same level behave normally
whether the issue predates or postdates other plumbing work

What to do next

A short, accurate account of what has already been tried changes what a professional does when they arrive.

Check what a professional will want to know

If your situation is different

Other fixtures are affected too: More than one drain affected

Drain chemicals
What has already gone down the drain is a safety issue for whoever attends.