I answer myself, hope it will help somebody else ...
@return true Returns true if message reached the **first stop ** on its way to destination.
indeed, it clarifies my question...

/**
	* Sends a message to gateway or one of the other nodes in the radio network
	*
	* @param msg Message to send
	* @param ack Set this to true if you want destination node to send ack back to this node. Default is not to request any ack.
	* @return true Returns true if message reached the first stop on its way to destination.
	*/
	bool send(MyMessage &msg, bool ack=false);

@hek said:

@doblanch said:

btw, you didn't answer my question, is there a trusted NRF/PA/LNA seller,

I cannot make any guarantees. But ITead should sell the genuine thing.

it was my assumption too ...
If I enable debug, will it provide me more info about communication ?

@hek

@hek said:

I don't think it should search...
https://github.com/mysensors/Arduino/blob/master/libraries/MySensors/MySensor.cpp#L463

But you might wanna try the development branch anyway to see if you notice any difference.

I can try at the next occurence... as I have no more idea... I think that code is clean, and problem is rather on the NRF. Maybe a starvation of something after a while, I don't know. difficult to say.
btw, you didn't answer my question, is there a trusted NRF/PA/LNA seller, with a tested NRF24 tested by you, and working at it should ?

@sundberg84 I use the last version (I guess nov 2015).
@hek it can be the case ? trying to use another route.
There is no other route available normally. I have disable repeater mode on all powered sensor.
Could it be a bad NRF24 ? is there a brand well-kwown really trusted ?

@hek ok Yes of course, I tried, it doesn't work...ceiling is really too thick !
12H03 - 12H23 We can see clearly the drops... and I have st=fail.
I resend packet based on st=fail, but resend is dropped also

@hek My antenna is powered externally directly in 3.3V with 4.7uF...
What is vanilla radio ? distance is 5m, but a big ceiling is in between...

@hek said:

Agree with you. Very strange. Especially when you use a direct route to gw.

the strange thing : I receive alsmost 19000 messages without issue.
When it happens for the first time, the issues are more and more frequent.
the NRF PA LNA provided by ITEAD should be good, no ? Which PA LNA are you recommending ?
One more thing, when problem happens, and recover, I tried to make a reboot from my controller, and the WS is not longer communicating. I have to power down/up to recover

@hek "Could the issue be between gw/controller?" ... I have a st=fail so it shouldn't be between controller and GW ?

@hek said:
I looked in the other sensors (sensebender at the same time). I have 5 sensebender, and there is no drop at this time. Packets are received every 5 minutes during this period. So, I don't expect a freeze of the GW.
I use JEEDOM.
I don't expect issue between GW and Controller ... but it can be a lead.
it difficult to troubleshoot controller. Because it's a serial GW, if packet doesn't arrive to the controller, I can not check at the GW level, as there is no way to connect.

@hek, as explained in the previous post, I enforced the next hop (GW).. so, it shouldn't used another route ?
gw.begin(NULL, 102, false, 0);
here is my config.

@hek Nope, I don't have any microwave running when I have some drops.
I don't have microwave oven at home. Just a TV. It was shutdown during events.
Th eweird thing, is that it doesn't work for 15 minutes. I loss 3 rows (3X10 messages) with 5 minutes interval.
It means that's the issue happens for a while. After, this event, everything runs correctly.
During the event, I have another sensors ( 5 sensors sensebender) which send correctly a packet. It means that issue is not on GW. And anyway, I don't have issue with GW.
I have a big ceiling to pass through. On GW and sensor, I use 7db antenna.
I don't understand why it works for 6 days, and stop suddenly for 15 minutes !

@sundberg84 . I forgot to mention, that the route is enforced to 0. There is then, no computation of route, and the route is direct. When I have st=fail, my packet doesn't arrive to my GW, as my value doesn't appear my controller. I use a Serial GW, if my packet doesn't arrive to my controller, I can not check more. As the problem happens suddenly, I can put a sniffer in the air also.