Rear derailer limitations

[d4rmsc]

The back derailer has 7 cogs, however the chain does not go through the full range, please help!!
It is a Carrera subway 1 with sti/rapid fire gears.
No matter how I adjust the h/l screws, b screw, tension, wire or anything, the derailer does not guide the chain through the full 7 cogs, but only 4.
I can adjust it so it goes from gear 1 to 5, or mid range (2-6) or from 3 to 7 but it will not go from 1 to 7.
Have not changed any parts on it, ie. new gear shifter or derailer or anything.
How can I get this sorted??

[cyclerUK]

Have you got the cable clamped in the correct place on the derailleur?
Undo the cable and it should lie in the slot under the clamp.
While you've got the cable undone, check the movement of the shifter.
Tension the cable with one hand while operating the STI.
Check to see if the STI goes through its full range.
Also move the derailleur, by hand, through the full range of gears.