Ok so I am learning 18 An Life by Skid Row. The problem is the intro for some reason I can do it perfect the first time through but then I can not do it again?

E |--------------------------------|--------------------------------|]
B |----5-----------4---------------|----5-----------4---------------|
G |--------6-----------4-----------|--------6-----------4-----------|
D |------------------------4-------|------------------------4-------|
A |4-----------4---------------4h6-|4-----------4---------------2---|
E |--------------------------------|--------------------------------|

C#m B Asus2 Bsus4
E |----------------------------------|------0---------------0-------|
B |----5-----------4---------------|-------0---------------0-------|
G |--------6-----------4-----------|-------2---------------4-------|
D |------------------------4------- |----2---------------4-----------|
A |4-----------4---------------4---|0---------------2---------------|
E |--------------------------------|--------------------------------|

Now here is were the problem occur's. The second time I start it my index finger which is barring the 4th fret all the way up from the 5th string, for some reason I can not keep the higher string's down. Is it just finger strength or am I not doing something right. I mean my index finger look's like i put it on a grill an grilled my finger I'm pressing so hard.Can you guy's please help me out with this an if it is finger strenght can you recommened some video's on building that please.
I find that when barring with my index, if I use more of the edge of my finger it works better. This way the pressure is coming from my wrist to hold the strings down, instead of pressing straight down with only my finger pushing down. It's kinda like I lay my index across the strings, and then torque my wrist slightly to the left (assuming you fret with your left hand). I have way more pressure this way with way less effort.