Need explanations on DirectFailed

I host Bolt Sessions on a Playfab Multiplayer VM and sometimes the client gets to directly join, sometimes it doesn't. When it uses relay i get these logs on the server :

[NAT Punch] Ping [EndPoint IPv4 <ip>] [30]

...

[NAT Punch] Ping [EndPoint IPv4 <ip>] [1]

[NAT Punch] Ping Request Cancel from Player 2

and on the client i get :

[EndPoint IPv4 <ip>] retrying connect ...

multiple times before it falls back to Relay connection.

I would like to know what I should do to always use a Direct connection (if it is possible) ?

Thanks