Any of the following can be behind it, and more than one can apply at the same time:

• Build-up on the inside of the pipe. Hair, soap residue, grease and other everyday household material gradually narrow the pipe, so it takes less to block it than it used to.
• Restricted drainage. If the effective diameter of the pipe has been reduced, normal use is enough to cause a blockage.
• An obstruction further along the line, beyond the fitting itself, which localised clearing doesn't reach.
• How the plumbing is configured - pipe runs, falls and connections all affect how well waste moves away.
• Wider drainage issues affecting more than one fitting.
• Something being put down the drain regularly that the pipe can't cope with.

We can't tell you which of these applies to your home from a description. Nobody can honestly claim a cause without looking at it.

Because restoring flow and identifying why the flow stopped are two different pieces of work.

Clearing a blockage removes whatever is obstructing the pipe at that moment. That's a genuine fix if the obstruction was a one-off - something went down that shouldn't have, it's gone, and the drain behaves normally afterwards.

But if the pipe was already partly narrowed by build-up, or the drainage was restricted further along, clearing the immediate obstruction returns the drain to how it was the day before it blocked - which is to say, close to blocking again. Normal use then does the rest, and a few weeks later you're back where you started.

That's the difference between a drain that blocked and a drain that keeps blocking. The second one is telling you something about the condition of the drainage, not about what you put down it last Tuesday.

These observations cost you nothing and make the visit considerably more productive:

• Is it one drain, or are several fittings affected?
• Is drainage simply slow, or does it stop completely?
• How long after clearing does it come back - days, weeks, or months?
• Are there smells as well? A drain that smells and blocks is a useful combination to mention.
• Any gurgling or unusual sounds, especially from a different fitting to the one you're using?
• Does it correlate with anything - a particular fitting, heavy use, or a certain time of day?

If smells are part of the picture, our guide on why a bathroom drain smells covers what that usually points to.

There's nothing wrong with clearing a drain yourself. Plenty of blockages are exactly what they look like, and dealing with one in ten minutes is a perfectly good outcome.

The calculation changes when it's the third or fourth time. At that point you're spending time and money treating a symptom on a schedule, without ever finding out why the schedule exists. That's usually the more expensive path, not the cheaper one.

And at that point, the question changes from "How do I unblock this?" to "Why does this keep happening?"

A plumber comes out, looks at the drain and how it's actually behaving, and clears it. You can book a blocked drain visit, so the starting number isn't a mystery. Some blockages are more involved than others - where that's the case, or where the plumber finds something that needs more than clearing, you get the price for that work before it's carried out.