In my personal experience, as well as advice I've collected over the years, alternate fingerings are essential up in that range for most baritones.
A works a lot better on 3rd.
C is a problem with no standard solution, but try 1st.
B is always flat on lots of different instruments, I use 2nd and lip up.
I would tune those slides to those notes and then see if the instrument feels more in tune with that setup.
You might be able to lip the notes into submission with regular fingerings but reliability is not great when lipping.
Last bit of suggestion - have the valve alignments checked. When I got a new Neo euphonium, the factory felts were way too thick.
Ted
Besson Prestige BE2052-8G-0 Euphonium
Besson Sovereign 956 Baritone