Hi Nic, there are rather more than three tenses in English. I'd suggest you stick an example of the switching in the Writing Groups and ask there for advice. With an example it'll be easier for us to see what the problem is.
Oh, I see. Is the example this one? I didn't see you asking there about tenses. Am I missing something? If your question is general, it's ok to talk about it here in Café Life, but if it's specific to a work in progress, it's better to chat about it in the Groups.