Is it normal an increase in pressure when diverter valve switch from waste to MS? if not, how to

Lisa-  The back pressure increase that you see when the flow is diverted to the source is due to the extra resistance of the nebulize and the tubing to the source being added in-line.   The change you see looks about right for that, but it is flow rate dependent.

I would suspect the HPLC column or maybe a void volume in one or more of the fittings is the source of the change in chromatographic performance.  Did the chromatographic performance change happen gradually or suddenly?  Maybe flushing the HPLC column with compatible, but more eluting or solubilizing solvent might help.  You may want to try a new HPLC column.

thank you for your answer. I flushed the column (reverse and normal), I checked the fittings, I tried 4 different columns (one brand new out of the box), I prepared freshly all the eluents and diluents using new bottles for everything and I still have the problem. I have no idea what to try next.
I checked the signal in DAD and the peak shape looks quite ok, the peak is from 2 min to 2.5 but in MS is from 2 min to 3.5. Could it be the nebulizer? I thought about some impurity but I used new glassware and new solutions.
I developed the method in March and now I have to validate it. In the meantime I didn't run it so I can't tell if the performance change was sudden or not 

Lisa- Some things to check:   Maybe replace the line from the DAD to the MS.  Double check your MS method settings and make sure they have not changed.  Maybe connect the line directly from the DAD to the MS nebulizer and skip the divert valve, if the divert times are not critical for you sample or standard.

What MS are you using? Can you share example chromatograms both UV and MS from earlier and now?

Hello, yesterday I tried to connect directly the DAD to the MS without diverter valve and I still have the same problem. MS parameter checked a thousand times :) and I also tried an old method for the same impurity I developed with a different gradient.. tailing also in that case. I thought it could have been something wrong with column or eluents but in DAD the peak looks ok. I have the problem only in MS in SIM and MRM. I tried in Scan and the peak looks ok but maybe is because of the higher noise?
Here the screenshots of development and validation. Unfortunatelly I don't have a DAD picture from development 

many thanks in advance

Hi Lisa,

Great suggestions from andyg. I would also try bypassing the side of the MS and go directly to nebulizer.