Session resumption is one of those features that is easy to enable and easy to forget. It saves a round trip and a signature, which matters on long paths, and it costs you a shared cache that has to be sized and expired on purpose.
Tickets are the usual choice because they need no server state, but they do need a key that is rotated on a schedule and not shared across more hosts than necessary.
Worth checking from time to time: that resumption is actually happening, that the ticket lifetime is what you think it is, and that a rotation does not silently invalidate every ticket at once.