I don't find that weird at all. Calves tend to be hide. Sometimes a cow calves and they hide them so well you can't find them until they come out and show themselves.
You do know that your "cull cow" will be entering the food chain right? So saying it's ok because it's a cull cow when it leaves isn't really a good excuse.
I think before you contact the seller you need to have the vet find out why your cow isn't cycling regularly? Something isn't right, and you need to know what it is if you want to bred her with the rest of your herd. Good luck :)