asterisk 8.14.0 and multicast sometimes not hear anythign

I am just doing a basic call in.

exten => 140,1,Answer
exten => 140,n,Playback(beep)
exten => 140,n,Dial(MulticastRTP/basic/239.168.4.90:3040//t(15))
exten => 140,n,Hangup

this works - but "sometimes" I get reports that "nothing" was heard.
Is there anything special to do for multicast ?
Any thoughts on why once in a great while nothing would be heard ?

Thanks

Jerry

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