For every customer there will be a maximum amount of credit that a company will set. They will add together all the credit limits (even if you aren't using them) on the basis that you could decide to 'max out' (I think that's the expression), and then decide whether they would be prepared to add to that credit limit.
Steps you can take then include:
(1) If you have any old credit cards you no longer need then close the account.
(2)Also if the credi card limits are more than you need then ask them to reduce them.
(3) plus all the thinngs others have suggested