API not returning data randomly

Question

I am using DX web API Backend Web API Service. I have a strange issue.

I have deployed the DX web api to IIS on a Windows Server 2019. Occasionally in Postman (and in client Xamarin app) when a OData query is made the result is null ie query is successful (200 OK) but no data is returned.

I then run the source project (on same above server and connected to the same backend db) in Visual Studio which then runs in IIS Express and api appears in Swagger. I then take the existing token from Postman and use it in Swagger and the data is returned. I then run the query again in Postman without any change (ie same query and token as before) and this time data is retuned in Postman too. What is going on and how can I fix it? As the same token works in Swagger it can’t be the token? What else could it be?

Thanks

Regards