Canon Speedlite 420EX with Rebel XTI & stock 18-55 lens--problems

[STBickerton]

I recently purchased a used Canon Speedlite 420EX for my Digital Rebel XTi with the stock 18-55 lens. Before purchasing, I confirmed with Canon that the EX series of flashes would be compatible with that camera. When I received it, I put new batteries (alkaline) and tried a test flash using the “Pilot” button but it would not fire (all buttons/switches were set for sole, non-remote use). I tried it on the camera but it also would not fire. The next day, I tried again and it would work. However, I noticed that it was most likely not to fire when the lens was set in the wide angle (18mm) position, and would not start firing until I set it up around 40 or 50 mm. Sometimes, however, it will flash at the 18 mm setting. Sometimes I can get it to flash at 18 mm if I take pictures at 40 or 50mm first then widen the angle to 18mm. Sometimes it will not even flash at 50 or 55 mm. I don’t know if this is a problem with the lens not being compatible with the flash or if it is an intermittent problem with the flash. Has anyone here experienced this problem? I would like some suggestions as soon as possible because I found a Quantaray QDC900WA flash on Craigslist, and can pick it up later this week​

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Thanks in advance for your help and advice.​