Thank you BigBoss for your answer!
Here I attach my layout circuit. From right to left there are: RF input (antenna), a switch which let me use an onboard BPF (you can see on the layout) or a external BPF (you can see only the ports). a switch again to go to the main path, a coupler, the LNA, a LPF and a second coupler which now is replaced by a 50 ohm resistor in order to find the LNA oscillation source.
Like I explained before, I have replaced and moved some devices in order to find the oscillation source. After some test, I could say my problem is when connect the first coupler (the one on the right, top view) to the LNA input. Why? because when I replace both couplers by a 50 ohm resistor (the LNA input is 50 ohm and the LNA output is the LPF and a 50 ohm resistor) the LNA does not oscillates. but, when I replace only the first coupler (with a 50 ohm resistor in its input, no RF signal) the LNA oscillates again.
I have tested others possible source like the supply voltage, biasing. But using a external supply voltage still the oscillation is there. Nevertheless, I make some test PCB to test the LNA, the LPF, the BPF. I connected all of them, and the result was the expected theoretically. The only way to produce the oscillation on the test PCB is connecting the output to the input. Of course then the LNA oscillates at the same frequency like my final circuit.
I am RF engineering student and I would like to know your opinion about my problem, and what you would do to fix this problem with, what I guess, my coupler problem.
If something is not understood, ask me i will try to explain again. Thank you for your help!