When you say water parameters are good, what does that mean?
What is the pH, nitrate, and hardness of your water, compared with the water they were shipped in?
Shipping is stressful for any fish, and if your parameters are drastically different than how they were raised (a very distinct possibility), it could be enough to cause problems,
Also
were the new fish put in a quarantine tank?
QT while usually thought of as protection for fish in your main tank
is also about gradually helping new fish adapt to whatever bacteria or parameters might be in the main tank.
Your old fish may have developed an immunity to something the new fish do not have immunity to.
I quarantine all new fish for a minimum of 3 months before adding to the main tank (community), gradually adding water from the tank they will go into, a little at a time.
In this way, the new fish develop resistance to whatever is in that water, and to the general water parameters.
This also protects your old fish from being subjected to a barrage of bacteria if the new fish get sick, overwhelm their developed immunity, and possibly start an new epidemic.
It often takes 3 months for certain diseases to present visually.
A friend who runs (ran)the aquatic exhibit at the Milwaukee zoo, Qts all new arrivals, for no less than 6 months.