Why is Ubuntu Software center so hit and miss - ?
Ubuntu user here - obviously

For example, right now I opened it, clicked the magnifying glass to search, and typed in "Gimp"

Well, it just searches and searches and twirls and twirls and nothing is ever returned.

And it's not just Gimp. It seems like 50% or so of software that I search for, often that I know is in there like Gimp, never returns a result - ???

I often have to resort to Synaptic Software Package manager to get stuff...

Don't know but 'sudo apt install gimp' seems to work just fine from the command-line . I am not a fan of the GUI way of getting software. If I need a piece of software and don't know how to install it, I do a quick web search and ... oh, there it is ... apt install <xyz> . GUI software managers always seem to be a bit flaky in my experience.

@kernelhead

If you are going to use a GUI software center then 'Synaptic Software Package manager to get stuff' is consistent no matter what distro you use and safe to use with options to opt out before final decision.

If you know the software name that you want to install using the terminal is a better way as @rclark says,
also look at the commands for searching software in the terminal,

in Firefox Ctrl+P (makes a PDF) the commands cheat page for apt install
You are using Unbuntu this is one there are many others online

https://www.cyberciti.biz/tips/linux...eat-sheet.html

takes a few times using the terminal to get used to it after you know how it's a better choice.
Don't fear the Terminal

Thanks everyone - marking as solved.