Building your credit score can take several years. My initial suspicion points towards the four new credit lines pulling down your average age of debt. Make liberal use of creditboard's credit pulls database to see if you have a reasonable chance at getting your application accepted. Also, if you weren't initially accepted by their auto acceptance computer program, call the number and ask a real person if they can accept you. You'll need to prepare reasons why you should get accepted, or certain bad debts should be overlooked or whatnot.
Basically do more due diligence on which cards you should apply to (if any) given your fico score, utilization rates, avg age of debt, etc. Additionally, every 6 months or so, you can call your existing credit cards to ask for a credit limit increase.
https://creditboards.com/forums/index.php?app=creditpulls
