The working theory of waterjet is high pressure water mixed with sand.
If one of the two components is not right, the machine can’t cut well.

1. high pressure water:
Is the pump losing pressure? (check the videos by this link:What is losing pressure)
Is the orifice bad? (Bad orifice will cause a very large water stream and can’t cut through the material. The orifice with red cap,normal orifice, usually lasts around 20 hours. The orifice with blue cap,diamond orifice, usually lasts around 500 hours.)
Is the nozzle bad? (The nozzle usually last around 200 hours. Bad nozzle has a hole much larger that a new one)

2. Sand
Is the sand wet? (If the sand get wet, it won’t flow fluently. This will cause the cutting speed so low. )

Is the air for the small sand hopper works good? Check the places in the below pictures. Make sure the air flow fluently.

Does the air pressure stay constant? It is showed by the air regulator on the left side of the table frame. (The air control the mini-sand hopper. If the air pressure is not constant, the flow rate of the sand will change. )