I was thinking about what people had said, and I had a couple of old tips, so I tried them all compared to a new one and I think it is that it's just ran out. One of the used ones also tasted metallic but two of them didn't, so I'm guessing I've just vaped it dry! So thanks for the help!
With regards to cost, I was smoking around 30 a week which was costing about £13.. The pack of 5 tips is £9 and they last me about 2 weeks.. I know there are cheaper methods, but after trying out the disposables this felt like a good next step and it just works for me. And I am still saving