Distillation speed slowly dropping

I got an little issue:

I have an rig set up offshore, the temperature of the distillation compartment is ~350°c but I’ve noticed the speed of distillation( here for diesel) starts at 5l, then slooowly drops down to .5 or even less

I am making enough crude oil since the main tank is filling up, I’ve got the temp, the oil, but despite this the flow of diesel is dropping? ( like, over an half hour approximately)

Any ideas ?