I can not send fax using Asterisk?

hi,

I am using asterisk , some of my users can not take their faxes over sip on asterisk sometimes calls drop or asterisk try "retransmission..." , but I can take that faxes to my mail when I add my email address to receivers.

what is solution for this situation?
it can be nat problem or firewall problem, but I looked them configurations seem properly.
I think problem depends on that users network or missing some appliances may be missing pri or there is a redirections on network or there must be multiplexer or carrier for multi calls or connections. I do not know...

can we use canreinvite= yes and nat= yes and qualify = yes , what is their effect exactly when the connection speed slowly.
I looked them I think their configuration is normal.

can you tell me what rtnetlink should be too ( this can be the out of range question) ?

thanks in advance