credit card companies can see your other ballance and total avalable so if you have (for example)

Credit £1000 > Ballance £900 then you are using 90% of credit avalable so they are less likely to give you credit. Also they can reduce there credit limits because you missed payments or are up to the hilt on other credit commitments.

Stop applying for credit and get the ballances down on your other cards then see if they will offer you a better credit limit. Also try to pay off more ballance from capital one to show you can afford it rather than always having a ballance.

Good luck