Is there any chance your drivers are wonky? I'd try a reinstall, before buying anything else, except maybe a backup mouse. You should always have backup mice.
Logitech still carries the G400S, but it's easy to see they'd prefer moving away from that more traditional design toward a more modular aesthetic and lots of flashy buttons to wear out. That particular mouse didn't even appear during my first look at their gaming offerings, as if it was "hidden." I had to find the Logitech G400S page through Google. And then there's the price. If you're ready to shell out another $70, they're a great design, no doubt. The G402 Hyperion Fury is similar, but less expensive. I'm still using the G400, but for how long? Shysters want over a $100 for one of those.
It's a shame the G400 or G400S aren't more ubiquitous, as that shape has been around for roughly two decades. Obviously, people like it. Logitech often makes no damn sense to me, especially where product design is concerned.
So, if you can afford to drop another $70, just get another G400S. There's really nothing quite like it.
On the other had, let me throw this out:
http://www.amazon.com/Etekcity%C2%AE-Scroll-M555-Programmable-Professional/dp/B00F3L19KQ/ref=sr_1_2?ie=UTF8&qid=1437077632&sr=8-2&keywords=Etekcity+mouse
I bought a few of the 2400 DPI model, and keep them for when my G400 dies. They were $13 when I last bought one. Obviously, you could buy several for the price of that Logitech G400S.
If you are right-handed, they are extremely comfortable. Your hand wraps around them very naturally. They're a bit longer than your Logitech mouse. The weight is fine. The build quality is fine. They just don't cost as much as the stuff we normally get steered toward.
I've never installed the drivers, but those would allow you to reprogram the buttons. Thing is, you're not going to reach the DPI switches up top during a fire fight. I've always reprogrammed the DPI switches on my Logitech mice as PGUP and PGDN. I didn't do that with this particular mouse, because they're not as easy to reach. And because I didn't want to install the drivers, in case I went back to Logitech. Which I did, eventually, because the G400 was pretty cheap when I bought it.
Why am I going on about that? Those top two or three buttons may be important to you. If they are not, I can HIGHLY recommend this Etekcity mouse, because it is more comfortable than the G400S. I appreciate your feelings for the Logitech mouse—I love that particular body style. My first one may have been the MX510. That body style is as close to perfect as the old Microsoft Intellimouse. This Etekcity is different, very much a right-handed design. While it is super-comfortable, those DPI switches up top will disappear between your first two fingers during normal use.
I'd like to see more gamers start to look beyond familiar brand names and explore products normally marketed for other parts of the world. There's some really neat stuff out there we never get to see. You could spend days kicking around Amazon, looking at all the groovy gaming mice from companies you've never heard of.
So, yeah, shell out another $70, or—if you don't normally use the upper buttons—try the Etekcity. Or get one as a backup. It's a really cool design.
Some other brands to look at:
Anker
Aula (some large models great for large hands)
Genius
Perixx
Redragon (some large models great for large hands)
Steelseries
Facebook
Twitter
Instagram