Probably inquiries. You'll hear that inquiries don't account for a lot but that's not exactly true. It depends on what's already on your credit report and how many inquiries you already had before you racked more up. That's what I've learned from my own credit journey.
Any significant rise in scores won't happen overnight but will happen sooner than you think. My scores were hovering around 600 when I first started paying attention to my credit at the end of March of this year. Now, all of my scores are above or very near to 700. At first I thought it would never happen but then I stopped obsessing over it, got back to life and it just happened with timely payments, of course.
There is some very good advice on these forums, however, be selective in the advice you take. You don't need a ton of CC's to jump start your credit score. A couple paid faithfully will be more than enough to get you started. Too many new TL's too soon can harm your credit score far more than it will help. If you must apply for new credit cards, do thorough research and only apply for the ones you know you have a real shot of getting. That way you don't waste inquiries and continually lower your score.
I hadn't applied for another CC since April until two weeks ago when my computer crashed and I had to buy another one. I was pleasantly surprised with a $2000 CL on a new card. It's my highest CL. Not bad considering I couldn't even get a CC a year ago and the two I got in April only had $400 ant $500 CL's.
