major problem in working with Serial1 port of MEGA 2560

I am currently developping an application to receive serial datas from electrical counter (teleinfo) in 1200 bds and I have checked that it was working fine with SoftwareSerial pin 10 of MEGA but impossible to get any data detected when using Serial1 (pin 19 rx1). I have an ethernet shield connected also...

note : I have checked that Serial1 was ok by using a simple test junction between a UNO as emitter in 1200 bds connected to the pin 19 of my MEGA and there was no problems with that... so the Serial1 is fine!

note that the pb is the same with Serial2 and 3 ...

I really don't understand why it is different when using my MEGA softserial port or the hard serial1 port : the test soft is very simple: only a (if Serial1.available () > 0) function is used but I never pass through it ...

I really wants to know if someone experiments something like this... thks for your help.