Why sometimes the MTU discovery by UDP never get a response?

[Bright Zhao]

Normally, if the tinc server’s MTU discovery response can reach the tinc client side, they always can negotiate with a certain MTU size, but the case is, sometimes, the client sent variety size of MTU probe packet to server, but the server never respond.

Sometimes, if I change the client side’s tinc port,  the server’s MTU response can received by client.

BTW, the above case are all regarding the tinc client is reside in a network behind NAT(not public routable IP)

Any thoughts regarding this? What probably the root cause for this? And what we can do in order to make MTU probe response always can be received by client?