The illustrations below are exaggerated to clearly show potential oil flow issues. 

Here is an illustration showing the proper minimum and maximum oil level height inside the burn pot of an oil stove. The oil stove carburetor (Toby DVR5) will maintain the oil level height that is embossed on the outside of the carburetor. When the carburetor is turned on, oil will flow into the pot until it reaches the same height as it is inside the carburetor.  The stove must be perfectly leveled to maintain the proper oil level:

If the minimum oil level is not achieved inside the burn pot (10mm) then fuel will not flow and the stove will not operate

If the maximum oil level is exceeded inside the burn pot ( closer than 17mm from the lowest vent holes) then there is a risk of oil overflowing out the vent holes. NEVER MODIFY YOUR STOVE TO RAISE THE FUEL LEVEL TOO CLOSE TO THE VENT HOLES.

The picture below illustrates how an unlevel stove prevents fuel from flowing into the burn pot:

The picture below illustrates how a hump in the fuel line or an upwards facing decoker can prevent fuel from flowing. The decoker should be oriented slightly downwards to prevent a high hump that the fuel cannot flow over:

The picture below shows that allowing carbon to build up inside the burn pot will create a blockage that fuel cannot flow over: