403 Forbidden Error Help?

Hey, so I am running some code for a project and it goes on the internet and grabs data from a site. My question is, about half the time i get a 403 forbidden error, but the URL does not change. I input the URL and it will go and fetch data, but sometimes the program works and others it returns a 403. So how can it be that sometimes I am forbidden and other times I have permission to access the page. Why is it not all or none? and if you think you know what my problem could be then help would aso be great.

Thanks so much.