The MaxProficiencies property's Setter does not always fire, even when I change the value. Why is it doing that?

Question

We're using MVVM Light. Here's the MaxProficiencies definition:

public int? MaxProficiencies
{
 get
 {
 if (Solution != null)
 {
 return Solution.MaxProficiencies;
 }

 IsMaxProficienciesCompleted = false;

 return 0;
 }
 set
 {
 if (value == null)
 {
 IsMaxProficienciesCompleted = false;
 }
 else
 {
 var tmpMaxProfs = value.Value;
 Solution.MaxProficiencies = tmpMaxProfs;
 solutionDirty = true;
 AvailProficiencies = CalculateAvailableProficiencies();

 if (value > 0)
 {
 IsMaxProficienciesCompleted = true;
 }
 else
 {
 IsMaxProficienciesCompleted = false;
 }
 }

 RaisePropertyChanged();
 }
}

I have a strong suspicious that this validation rule has a lot to do with the Max Proficiencies Setter not firing the second time Max Proficiencies is changed:

public class MaxProficienciesValidationRule : ValidationRule
{
 /// <summary>
 /// The minimum integer value that MaxProficiences can be
 /// </summary>
 public long Min { get; set; }

 /// <summary>
 /// The value of the current Solution ID
 /// </summary>
 public long SolutionID { get; set; }

 public override ValidationResult Validate(object value, CultureInfo cultureInfo)
 {
 if (value == null)
 {
 return new ValidationResult(false, "Empty string invalid");
 }

 if (SolutionID > 0)
 {
 //this is an existing Solution, so it must have been previously validated
 return new ValidationResult(true, "");
 }

 long proposedMaxProficiencies;
 try
 {
 if (((string)value).Length > 0)
 {
 proposedMaxProficiencies = Int64.Parse((string)value);
 }
 else
 {
 return new ValidationResult(false, "Empty string invalid");
 }
 }
 catch (Exception ex)
 {
 return new ValidationResult(false, $"Illegal character(s) or {ex.Message}");
 }

 if (proposedMaxProficiencies < Min)
 {
 return new ValidationResult(false, $"MaxProficiencies must be greater than or equal to {Min}");
 }

 return new ValidationResult(true, "");
 }

 public MaxProficienciesValidationRuleWrapper Wrapper { get; set; }
}

And finally, here's the code snippet from the ViewModel, where I've defined the MaxProficiencies property. I've placed a breakpoint at the start of the Set. If I enter in a value, say 1. It fires. However, if I clear the value, from that point onward it no longer goes to the Set, even if I put in 0, or 1, or 10. It just stops every being set again.

public int? MaxProficiencies
{
    get
    {
        if (Solution != null)
        {
            return Solution.MaxProficiencies;
        }

        IsMaxProficienciesCompleted = false;

        return 0;
    }
    set
    {
        if (value == null)
        {
            IsMaxProficienciesCompleted = false;
        }
        else
        {
            var tmpMaxProfs = value.Value;
            Solution.MaxProficiencies = tmpMaxProfs;
            solutionDirty = true;
            AvailProficiencies = CalculateAvailableProficiencies();

            if (value > 0)
            {
                IsMaxProficienciesCompleted = true;
            }
            else
            {
                IsMaxProficienciesCompleted = false;
            }
        }

        RaisePropertyChanged();
    }
}