Yes, this is sometimes a problem with the NMR's. There are three different types of probes with 3 different field ranges. Sometimes the default probe doesn't work as well as an alternate. Probes A and E are for the same field range (0.17-0.52 T) Probes B and F are for the same field range (0.35-1.05 T) Probes C and G are for the same field range (0.70-2.10 T)
IF YOU HAVE TROUBLE GETTING A LOCK TRY ANOTHER PROBE.
PROCEDURE: With regulator ON, set P0 If after a reasonable amount of time (This varies greatly depending on whether the new P0 is very different or similar to the last one. If the magnet current has stabilized that's a reasonable amount of time) and you still have no lock or a weak one, on the NMR control screen switch to a different probe. You can tell from the Hall Probe reading which ones you should try. N.B. If you turn the regulator off and then turn it on again or change the P0 setting you will have to select the alternate probe again, assuming you're staying in the same field region)