no unicorn foal?

There is no VIP perk that affects the 6th foal guarantee, it's always every 6 foals. The VIP perk just increases your chances fro 20% to 25%. So if it's only the 5th covering, the next one will be a unicorn.

The mention of the coverings resetting after she's sold is also not true, she can be covered by previous owners and this will not affect it.

However, what WILL affect it is number of foals, and whether she was a mare for all 6 coverings. If she was bolted (a stallion for some of the covers) any covers made from when she was a stallion do not count. Twins also do not count as two foals. If some or all of her covers were twins, she may not be on her 6th pregnancy.

The other thing that can happen is possibly she had a unicorn foal, but it was dehorned and now shows up as a horse on her offspring page. Check through some of the prior foals and see if they have an objective bonus. If they do, it's a former unicorn.

The last possibility is that she had a unicorn foal that died or was put in the Safe Haven and now no longer shows up on her page.

here's a question that I don't know the answer to. do all servers have the same rules like the 6th uni being guaranteed?

As near as I can tell, the mare has to have been foaled after a certain date (whenever the new rule went into effect). This mare looks to be foaled in 2020, which would be before that date.

You really should make your own forum question but yes, I dont see why they wouldn't? Why would howrse create such a specific game detail and not allow it on some servers?
I get there are region specific breeds on some servers but that doesn't affect gameplay like the genetics/breeding would. Why would howrse create a new system and not put it on all servers?

the answer to my question could be the problem. I know some of the really small servers don't run like the big ones do (like different forum setup) that's why I asked

As far as I'm aware all servers should have the same changes since recent years an effort was made to synchronise them.

We cannot see the mare and understand forums are slow on small servers but can you not ask your Contact Us?

Info given by Crookedstar915 is all good.