I could obviously not second guess either your experience or the check airman’s judgment, having not been there but....
Generally Vls in the scenario you describe is treated as a sort of “caution range” before you start hitting protections. In other words, if you wind up there it will prompt you to very quickly get yourself out of whatever situation made you so slow in the first place. Basically being below Vls is a symptom of something else, whether turbulence / shear, or mishandling of some kind. The questions I would have would be:
How did you get below Vls? Rapid over-pitching? Mishandling thrust or rudder? Gusts?
How long were you there, how prompt were your corrections, and did you correct whatever originally caused the excursion?
The difference between an acceptable and unacceptable grade often has to do with how quickly you recognized the deviation, if you recognized what actually caused it in the first place, and importantly how timely and appropriate your correction was relative to the underlying cause (for example the pitch excursion that caused the speed to drop in the first place). It often also has to do with the “big picture” of the overall session, or the other aspects of the maneuver. Where there other issues that were close to the edges of the standard? Or was absolutely everything else perfect except for that one thing? Big picture context matters.
Somewhere your company has a standards document, which is likely based on and expanded from the ATPL test standards of your country. The company document should also contain specific guidelines for how to grade deviations from that standard. First thing I would do is ask to see this, and very, very honestly assess yourself based on what it says.
It is more important to see ourselves clearly than to be right all the time. Otherwise we lose the open mindedness to continue improving our own weak points, which we ALL have. I have been on both sides of your scenario, and let me tell you.... Ego has no place in this business. Yours will be bruised, just as much as mine has been! Your check airman has had his share, too.