A low point drain that is not functioning, not flowing groundwater out of a flooded crawl space, is probably plugged to the degree that it is no longer functional, if it ever was, and likely cannot be… cleaned or rooted out, as it probably is not plumbed to anything at all.
Not many low point drains are installed well enough to work however, and do not flow groundwater from the crawl space at all.
The low point foundation drain is a length of 3″ abs black plastic pipe, poured into the foundation when the home was built, with a flapper gate attached as a back flow, which seldom works well, if at all.
The low point drain may not exist at all, or is actually just back filled with dirt on the outside of the foundation, and not even plumbed to a vent, such as a dry well or day lighted vent.
The abs pipe poured in the foundation was likely not connected to anything to vent it on the outside of the foundation wall, and was probably just backfilled with dirt, when the home was built. Builders get away with that one often. This is often the case.
Home inspectors sometimes fuel this low point drain misunderstanding phenom with ignorant mutterings about low point drains in their home inspection reports, like, “adjust the low point drain”.
Adjust the low point drain? It is a piece of solid abs pipe poured too high in the foundation wall to work at all. And that is if one exists at all.
When the foundation is poured, the stem walls and footings are poured first. On the base of the stem wall, the 3″ abs low point drain pipe is poured in concrete.
As the bottom of the crawl space gets pounded down from workers walking on it, during the floor joists being installed, the low point drain sinks in grade, and from that point on, if it did previously work, will not work at all until many inches of groundwater has filled the crawl space.
The floor of the crawl space has been compacted lower than the low point drain. Oops. But this is an oops, that happens over and over again.
As a result, the bottom of the low point drain pipe is above the floor of the crawl space, invalidating the pipe as an effective low point drain, making it necessary to have 2-3 inches of groundwater standing over the entire crawl space floor, before any groundwater will start flowing out of the low point drain pipe. And that is, if it is even plumbed to a vent, which many are not. This is especially true of older homes, which often have no home drainage infrastructure protecting the home at all, and is also especially true of cheaper entry level ranch homes as well.
The groundwater solution is, most often, if you can, to hand excavated french drain groundwater removal systems, on the outside of the foundation wall, about 18″ from the wall, to stop the groundwater saturating against the foundation wall, which causes groundwater hydrostatic pressure and leaking below grade in crawl spaces.
Sump pumps even do a poor job at what they are designed to do, in my opinion, and many sump pump installers only do one thing, and that is done poorly.
Any necessity for, and results from the use of sump pumps is predicated on the professional installation methods studied and used, and how well the sump pump installations are set to discharge, and how often they go off, as well as how the groundwater within the crawl space is drained to the sump well area itself, such as in a crawl space french drain system, so it can be pumped and discharged.
Even then, pumping for life, your crawl space will always be wet, to some degree, which is not home drainage success by professional standards.