Tooth problems

Hello,

I have had two root canals with crowns and one crown with no root canal - one is perfect the other two are causing me concern. The first of concern has had root canal and a crown and sometimes is sensitive to pressure. It can be like this for a day and then not again for weeks. The tooth most recently crowned due to a hair line crack (only seen via xray) has become cold sensitive and pressure sensitive since the crown has been put on - the tooth was not cold sensitive prior to dental work nor did it suffer as much from pressure. Their is no aching from the tooth ever it is only unhappy to cold food/drinks and eating something medium-hard. It has been at least 4 weeks and is not getting any better - why is this??????