C# Dictionary ContainsKey/TryGetValue not working

Question

I've looked around for a while and seen plenty of references to modifying GetHashCode() and things when playing with ContainsKey() and TryGetValue() - but all those issues and examples have all been with some obscure user-specific key.

I have a basic Dictionary<long, object>. When trying either ContainsKey() or TryGetValue(), it doesn't always get a hit (when it should) and moves on to attempting to populate the missing entry (as it technically should if that were the case).

You guessed it: it then complains about the existing key because, well, it exists.

So why is it having problems matching a basic long key and how do you make it behave?

Edit: Code. I have tried several things. In their most basic form:

public void Add(long id)
    {
        if (AreaDataDict.ContainsKey(id)) return;

        AreaData ad = new AreaData(id);
        ad.Load();
        AreaDataDict.Add(id, ad);
    }

Also:

public void Add(long id)
{
    AreaData areaData;
    if (AreaDataDict.TryGetValue(id, out areaData)) return;

    AreaData ad = new AreaData(id);
    ad.Load();
    AreaDataDict.Add(id, ad);
}

Edit 2: No key changes are happening. Other than adding if the value is not found, data is only read from this.

Answer 1

Using a ConcurrentDictionary was the answer, thankfully inspired by user454076.