Think through the problem. If you have a peristaltic system, go from the pinch roller toward the handpiece. Is the pinch roller mechanism working properly? Is the line collapsed? (Shouldn't be, the tube wall rigidity should be enough to retain shape at max vacuum.) Is the line seal intact at the handpiece? Is the tip fastened correctly? Is the tip of the needle actually in occlusion in the nucleus material? Vacuum only works in full occlusion.
If you are working a Venturi system, is there a fault code on the machine? The other considerations apply forward the machine to the handpiece.
The equally important consideration is how to proceed with the case once a cause is identified that can't be repaired on the spot or if no cause can be identified and the vacuum problem can't be resolved.
That is where the value of complete training in all methods of nucleus extraction is essential, not just phaco. I am grateful to have learned the phaco-trisection manual small-incision technique. You could salvage a case like this by that technique.