There are several possible causes:

The amount of engraving medium in the machine may be too little or too much. The ideal level is when flattened out, the medium should be about level with the nut that holds the pick up pipe into place. (the pick up pipe is located down the right hand side inside of the hopper).

There may be a hole, or holes somewhere in the pipework between the engraving gun and the pick up pipe, or even in the pick up pipe itself.

The pick up pipe may be in the wrong position. Loosen the nut which holds the pipe into place, then adjust it up or down until the flow improves. Also, the pick up pipe is cut off at an angle at the bottom. The angle should face into the centre of the hopper.

There is a thin plastic pipe which feeds into the bottom of the hopper. This agitates the abrasive in the bottom of the hopper to allow the abrasive to flow freely into the pickup pipe. The valve outside the machine on the right hand (when viewed from the front) controls the airflow to the thin plastic pipe. Ensure this valve is turned on to allow this agitation to take place.

Make sure the abrasive in the cabinet is not damp as this will guarantee spitting.