I have had the same issue with a few fountain pens, I am not a great believer in shaking or splashing and being in a hurry to get them working.
Pressing hard on the nib is another one I have heard to get ink flowing.I just leave them standing on a bit of paper for a few hours, it soon starts working f its own accord.
Of the pens I have shifted given the same advise and nobody has come back complaining.
I never ink them up before getting rid, let the punter do that for themselves, that way they know they have a new pen with all the problems it may bring.