composite put on my left incisor goes a noticeably darker shade in weeks?!

Please can you advise me. I had a partial composite put on my left incisor. It covers only about half the tooth. I was advised that only this amount could be covered with the composite since the tooth is not as flat as it should be for a complete composite covering the tooth's surface. It looked reasonably white to match my other teeth, but it has only been about a few weeks and the tooth with the partial composite is a very noticeably darker shade. Please note I do not eat or drink easily staining food or drinks and I do not smoke or do anything to make it discolour so quickly.
I have had a partial composite on it before with another dentist at another establishment and it stayed the same shade for around a year. Please let me know what it could be? The dentist used an inferior material for composite? or a step was missed out in the process of adding a composite? Is it advisable to speak to the dentist about it, and get him to do it again properly? my email is nixodian@yahoo.co.uk and would so appreciate you informing me asap.


thanks in advance