How would you deal with a client even after fulfilling the requirements,but the client still doesn't think that it is fulfilled?

This is where clear requirements documents come in handy. In order to avoid any disagreement, I cannot emphasize enough that it is paramount to have signed off requirements documents before you implement them.

Even after having such clear requirements documents if the client disagrees, it would be prudent to discuss with your client and explain that you have implemented what was agreed upon in written requirements documents. You may incorporate the client's additional requirements as new requirements.

I think there is something lacking for which the client is not getting satisfied we have to see to it that we constantly improve our knowledge and communication skills and try to convince