Are you using the 868DM, or 868LP? Since you are using API mode, are you getting a TX Status response showing transmit failure?
I’m not sure duty cycle is the right wor - you seem to be estimating load or bandwidth utilization.
If you are using the older high-power 868LP, that one has problems if your nodes are too close together - I was told they had to be at least 5-6 meters apart or the RX circuits over-saturate.