LTC3124

Is the part set in forced PWM or burst? In another word, how did you set up the PWM/SYNC pin? If it is tied to high, the part will be in forced PWM mode, otherwise it will be in burst mode. Can you upload your schematic as well?

The part is set in forced PWM. 6V_PWM signal is always to "1" logic level

The part is set in forced PWM. 6V_PWM signal is always to "1" logic level.

In this case, the circuit is not running in the burst mode. The circuit will run in the burst mode only if PWM signal is set to low. At no load condition, the switch may skip pulses even if the part is set at PWM mode.

I don't understand why SWB is in skip pluses and SWA is disable. According to the datasheet SWB is disable.

When I applied a load, the circuit falls in soft start mode before to be in PWM mode. The output voltage decreases because it can't provide enough current in soft start mode.

Your circuit is running at forced PWM. The 1MHz switching frequency is correct in the waveforms you've shown.

The SWA turning off for a time may have something to do with this statement in the datasheet:
In non-synchronized applications, repeated clocking of the PWM/SYNC pin to affect an operating mode change is supported with these restrictions: • Boost Mode (VOUT > VIN): IOUT < 3mA: fPWM/SYNC ≤ 10Hz, IOUT ≥ 3mA: fPWM/SYNC ≤ 5kHz