I just replaced the valve and regulator on my old Q100 today. Part of the process requires removal of the burner tube, so, cleaned out the bottom while it was out. I did discover that placing the burner tube to close to the ignitor will cause it to not work. I'd suggest loosening the tube and sliding it slightly away from the ignitor for a larger spark.
Of course there must be a spark first!!!! I could see mine by covering the small hole to block light and looking toward the ignitor while pressing the button. Sucker has worked since new in Oct. 2005. Replaced the valve assembly just to prevent trouble as I did not like fluid that came out of hose before purchasing a new 5 lb. tank. Low flame was concerning me.