Why does it different?????!!!!

Uuuh...
Hi. Welcome to talkbass.

When you're first starting out, chances are you're not too accurate with that thumb when it comes to slapping. I know I wasn't when I first learned how to slap, but slapping the E string was easy. This is most likely due to the fact that there are no strings lower than the E on a standard 4 string, so you don't have to worry about hitting any other strings.

The problem arises when you try to slap the A string, right?
First thing you have to do is make sure you're hitting the A string and the A string only. Your thumb might brush against the E, but it should only really be hitting the A string. To compensate for this brushing against the E string, you might want to try muting it some how, either by wrapping your thumb around the neck and lightly touching the E with your thumb (Although many people here will tell you that's "wrong" or "bad", and...well, really it is, if you want to keep your wrists and arms healthy. ) My preferred method of muting the E is to bring the middle or ring finger on my fretting hand up to mute the E string. It bugs the heck out of other people trying to figure out how I do it, because they always mute the string they're slapping, too.

Anyway, like I said, you'll want to mute that E string and make sure you're only really hitting the A string.
Try and pretend that the E string isn't even there. That could help, too.

 

Volume should be the same from string to string if you use the same amount of strength.

As a general rule, though, you shouldn't have to use too much strength anyway. If when you slap the A string you don't get as much volume as you do with the E, then perhaps it's more of an accuracy thing, like I said in my earlier post. However, the easiest way for anyone to help you is to hear and see you do it, so they can find out what exactly it is that's causing the difference in volume levels, if it's even related to your technique.

Unfortunately, this is where my help stops, seeing as I'm pretty new to the bass myself. I might be able to help you a little more if I could see and hear you slap the E and A strings, but that doesn't seem at all likely, unless "I" is "Hong Kong" in the location portion of your profile. So...I guess I'll have to ask, where abouts in the world are you?
Did that sound right?

 

Well, the A will naturally sound a little less "fat" than the E since its lighter and tuned higher, but the volume should be much more even between E and A than , say, A and D.

Make sure your bass is setup so that your bridge/relief is set even between all strings. Now, if you still don't get even volume, you're probably not doing it right... keep going at it...bounce off the string...practice getting such a clean sound as possible...

Good luck,
/lovebown

 

8.5??

For slap you should probably lower it to atleast 5 mm in my opinion, personally I go for about 2,5
/lovebown