Weak vacuum at the nozzle prevents location teach.
The vacuum values on the I/O and Sensors Diagnostic dialog are a function of the vacuum transducer. If the vacuum is weak, the transducer will not change much, and these values will change little as well.
What is the reading on the vacuum gauge on the pneumatics panel? Less than 20 in/Hg is sign a problem. It could be a leak in the vacuum lines, a defective vacuum generator or vacuum valve. If the vacuum generator is good, go up to the head and pull the vacuum hose off the curved metal tube on the Z rod, and check the vacuum at the hose. If the vacuum is strong there, but weak at the nozzle, then the leak is inside the Z rod, probably the V-rings. If the vacuum is weak, the problem could be the in-line vacuum filter or the vacuum valve. Finally, there could be a split or cracked hose in the e-chain.