The motherlode is in a very unstable area. Even relatively small excavations cause the surrounding dirt/sand to collapse, so for every 10 ft (for example) you dig towards the lode, another 8-10 ft fills in your hole. And normal methods for holding back collapses don't have anything to anchor to, so they end up shifting in dangerous ways or simply collapsing, too.
Since you are looking at technology around 1800, there's little in the way of large industrial machines, so most of the digging is by hand or rudimentary machines, so it simply takes too much time and effort to dig, at least compared to the easier to access ore under water. Mining machines exist, but are rare and expensive. They won't become common until the later half of the century.
https://www.calaverashistory.org/mining-methods
Yes, you can throw more people at it, but that gets expensive fast, especially since you don't actually know where the lode is or even how large it is, and you aren't getting anything out of the excavation. You've gotten investors before, but they quit sending good money after bad.
You've also lost a considerable amount of workers in accidents due to the unstable landscape, so people are wary of the job site and you have fewer and fewer people at the mine every week, so you can't even hire more people if you wanted to.
Dredging has been around since around 1575, so the technology is fairly mature for your needs. And it's pretty quick, at least compared to the alternative.
https://www.royalihc.com/dredging/history-dredging
You've sent divers to check out how much ore is available. They don't have SCUBA gear, so they can't stay down super long, but they tell you that the ore is spread far and wide. You've also done core samples at random places, which shows that the bottom of the river delta has a very thick layer of ore, so you don't need to go through the expense of digging out the motherlode anytime soon. Your engineers assure you that it'll be decades before you come even close to running out of the easy to access ore.
Alternative options
Other issues that can come up is that the motherlode is in another jurisdiction than the mining community. Or they simply don't have access to it.
Maybe a competing company owns the land, but doesn't know about the lode, and the company doesn't want competition by bringing attention to it.
Maybe it's in a community that forbids commercial mining.
Maybe it's owned by some rich people that want to keep the view from their home pristine, rather than have a large strip mine tear down the trees.
Maybe there's a local native community living on it. Either they are hostile and no one comes beck from trying to negotiate, or they are extremely beneficial to the community and no one wants to risk their generosity. Maybe it's on land sacred to the natives and they guard it ferociously regardless if they are normally amiable.
Maybe there's already significant housing or a thriving industrial section on it and no one wants to tear it up to get to the lode.
Maybe there's public resistance to tearing down a well known landmark.
Maybe if the motherlode area is damaged, the river dries up or the river alters course.
I'm sure there are many other alternatives that I haven't mentioned.
Multiple ores
In order to pull out multiple types of ore, you need chemistry. Most of the stuff pulled out of the ground is mixed with something else. Ancient Egyptians were able to get gold to 95% pure, but whether they were able to use the silver, copper, and other impurities they pulled out of it, I don't know.
"The alloys used to produce the studied jewellery[sic] range from high purity ones, with a gold content that can reach 95%..."
https://journals.openedition.org/archeosciences/2095?lang=en
Even more recently, slag from coal mines are now being processed to get minerals from them that was previously ignored or unable to be accessed, but this wasn't done until fairly recently, but I can't seem to find any dates in any articles I've found about the process.
https://www.sciencedirect.com/science/article/abs/pii/S1383586609001828