Reconnecting players are being throttled
What it looks like in your log
Connection throttled! Please wait before reconnecting.
What causes it
The same address reconnected within the connection-throttle window in spigot.yml, which blocks quick rejoins.
How to fix it
Raise or disable connection-throttle in spigot.yml (set it to -1 on backends behind a proxy).
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log