Updated: May 20, 2021
It's generally expected that if flow stops, you've got a clog somewhere, but what about the times everything is working and then the water just trickles down to nothing?
You stop, adjust the controls check your equipment and then things are back to normal, but only for a little bit. This is the more common presentation of a clogged line.
Usually, clogs don't completely block flow, they just obstruct it so when at rest, pressure will build up in the lines and you'll have normal performance, but only until the built-up line pressure is exhausted.
It's analogous to a dripping faucet. If you stop the drain, you'll fill the sink eventually. But as soon as you pull the plug, the sink will drain and you'll be left with just a drip.
Just like a dripping sink, when not in use, pressure builds up in your lines, but as soon as you start to use the unit, pressure drains and can't be maintained as there is so little flow.
Unfortunately, once you've deduced you've got a clog, there's no simple way to find it. It's time to simply trace the lines to find out where the clog is. Every connection is a point of restriction as are any "T" junctions. Check every one of them.
Often, it will be a matter of looking for where you have good flow. The last point you checked prior to that is where you have your clog.