Straddle Splits! Havent been able to do those for years -_-.
The problem may not be meerely the routine your doing, but the ammount of time your spending on it for the Straddle it's commonly advised to increase the actual hold or sitting of your stretch rather than the routine.
However a few good stretches that got me there when I was younger were
High kicks..10-15 of them(each leg), then go down into your splits hold for 2 minutes, repeat hold for 3 , repeat hold for 4, repeat hold for 5.
The best however are isometric stretches(the only downside being you need to find a partner willing to work with you for about five minutes+)
Lie on your back leg in the air over a shoulder of a partner and have him push against, relax, push against, rleax etc.
Make sure your muscles are warm before you stretc(sounds silly since most people stretch as part of a warm up) and when your doing your Straddles themselves make sure you position and back poise is spot on.