Phaser 7400 black streaks imaging unit needs replacement too soon

[AndrewUSOG0116-xrx]

Greetings, 

There are many factors that can modify the expected lifespan of the toner, imaging unit, fuser, rollers, and other consumable parts. If you would like further information on the matter, I recommend that you call into phone support to speak with a technician on the subject. 

Thanks,

[AbbottCards]

Six months ago I replaced all imaging units on our Phaser 7400 (black, cyan, yellow, magenta).  These units have previously lasted 18-24 months on this machine.  Black streaks are appearing on print outs and our printer technician believes we need to replace the black imaging unit.  The imaging unit installed 6 months ago yields 30,000 pages and we are nowhere near this usage.  Why would a black imaging unit run out so quickly???