8gb installed available 4gb

Whats your mobo?

Do u have it installed in correct slot?

My mobo is Asus P7P55D-E Intel P55 (Socket 1156) DDR3 Motherboard

And yeah there in the right ones

Boot with (1) stick then swap the stick; it's faster to rule-out a bad stick.

Q - What CPU?

You got a bad stick of RAM

will try that now, i use a intel i5 760

Ok so ive tried it and it boots up with both of them individually, the only difference is that the 1 stick wont boot in the first slot unlike the other stick which will boot in either of them if that helps....

No, 1 stick should use the 2nd DIMM slot from the CPU; DIMM_A1

Just to check, use only the following DIMM slots:
CPU: | blank | 4GB | blank | 4GB | ; Blue DIMM slots only

Intel Core i5-760 2.80 GHz

Assuming - CORSAIR XMS3 8GB (2 x 4GB) DDR3 1600 CMX8GX3M2B1600C9
9-9-9-24-2T @ 1.50v

Assuming - there's no bad stick.

BIOS:
AI Overclock Tuner -> Auto
DRAM Frequency -> DDR3-1600
DRAM Timing -> Manual
DRAM CAS -> 9
DRAM RAS to CAS -> 9
DRAM RAS PRE Time -> 9
DRAM RAS ACT Time -> 24
-
DRAM Timing Mode -> 2N
-
IMC Voltage -> 1.20v~1.25v
DRAM Voltage -> 1.50v~1.55v

If the problem persists then IMO update the BIOS to Version 1504 using ASUS EZ Flash 2:
EZ Flash 2 is a built-in App to allow the BIOS to be upgraded and USB Flash Drive is the storage for the BIOS ROM file to be stored/accessed from with BIOS EZ Flash 2. The USB Flash drive must be formatted in FAT-32.

Link -> http://vip.asus.com/forum/view.aspx?id=2007021522310966...
Video -> http://www.youtube.com/watch?v=NQKQj_aASCQ

Yeah ive been using those slots will go check now... thanks