Hi Enzo,
As Mark has pointed out, you need a fair ERP to get in to the mode L receiver. I have about 2500 ERP, and can get in fairly reliably with no trees in the way, but not as well as I did back in the KO-23 days. Because of antenna placement (?) it sometimes makes a big difference whether the satellite is approaching or leaving, both on packet and mode L voice.
Two things to check are your frequency and calibration, and your deviation. Not all rigs, and especially transverters, are accurately calibrated at that frequency. Mine are not, and I used a laboratory grade frequency counter to get the correct values.
Also, there is a consensus that the actually uplink frequency is about 3 KHz higher than published. I use 1268.703 MHz. The receiver is fairly wide, but every bit helps.
Finally, make certain the 9600 baud deviation is good, preferably with a good deviation meter.
Good luck,
Alan WA4SCA