Vax Vacuum Mystery!

HI, I have a Vax upright vacuum cleaner, model U89 Ma Te. When used as a regular carpet vacuum it works perfectly. But when I pull out the telescopic extension tube to use any of the tool attachments, there is no suction whatsoever. I have tried everything I can think of. I have checked for blockages, checked the hoses for any splits or holes, checked the filters. But everything seems fine. What is baffling is that when I put the extension tube back into its normal position and continue normal vacuuming of a carpet, all works perfectly with brilliant, powerful suction. Does anybody have any idea as to what the problem may be? Thanks.