Suppose, not hypothetically, that you have two modules, such as a program and a general module that it uses. Through working on the program, you realize that there are some bugs in the general module, so you fix them and then test them in the program by temporarily using a replace directive, or perhaps a workspace. Eventually you're satisfied with the changes to your module, so you commit them and push the change to the public repository. Now you want to update your program's go.mod to use the module version you've just pushed.

As lots of instructions will tell you, this is straightforward; you want some version of 'go get -u', perhaps 'go get -u .'. However, if you try this immediately, you may discover that Go is not updating the module's version. No matter what you do, not even removing the module from 'go.mod' and then go-get'ing it again, will make Go budge. As far as Go seems to be concerned, your module has not updated and the only available version is the previous one.

(It's possible that 'go get -u <module>@latest' will work here, I didn't think to try it when this happened to me.)

As far as I can tell, what is going on here is the Go module proxy. By default, 'go get' will consult the (public) Go module proxy, and the Go module proxy can have a delay between when you push an update to the public repositories and when the module proxy sees it. I assume that under the hood there's various sorts of rate limiting and other caching, since I expect neither the Go proxy nor the various forges out there want the Go proxy to query forges on every single request just in case an infrequently updated module has been updated this time around.

The blunt hammer way of defeating this is to force 'go get -u' to not use the Go module proxy, with 'GOPROXY=direct go get -u'. This will force Go to directly query the public source and so make it notice your just-pushed update.

PS: If you tagged a new version I believe you can hand edit your go.mod to have the new version. This is more difficult if your module is not officially released, has no version tags, and is using the 'v0.0.0-<git information>' format in go.mod.

PPS: Possibly there is another way you're supposed to do this. If so, it doesn't seem to be well documented.