Buck converter stops working with higher power
I suspect that your inductor's core is saturating. 

You have an inductance of 1 mH and a switching frequency of 100 kHz and this tells me that you might be at the point when all the energy put into the inductor is not being delvered to the load per switching cycle i.e. you operate in CCM (continuous current mode). Operating in this mode is fine but as you increase your supply voltage the average current in your inductor can rapidly increase and saturation of the core follows. When this happens the inductance falls to a very low value and creates the likely problems you see.

Another point; operating the MOSFET as a source follower DOES NOT make a very efficient buck regulator and that MOSFET will get hot and the whole point of a buck regulator is that it is an efficient design on load. Look at the pink trace - you are only getting a peak of 5 volts coming from the source - a decent design would be close to Vin or 9 volts as per the scope picture.