Scanning is as much an art as a science. There is a myth that scanning can easily be just a one button push process that results in great scans. There has been very little advancement in scanner technology for a number of years. If you are getting dull scans, it is most likely due to software settings or things not being set up to calibrate correctly. Additionally, scans that are initially and intentionally flat are often actually preferable because that usually means the maximum amount of data is there which gives you more working room in Photoshop (e.g. nothing is clipped). I would rather have all of the data there and then do the tweaking in Photoshop where I have more control versus the cookie-cutter approach that the scanning software can apply.
My guess is that maybe your scanner needs lubrication and that it is straining. If true this will eventually cause failure of the motor and/or gears. The most common areas needing lubrication are the sliding rails. I would open up the scanner and try a very minimal lubricating of the rails using a lubricant that has minimal vapors or outgassing. Clean the underside of the glass while you have it open. You will probably find a film on the underside from the outgassing of plastic over time.
I might try a poke around, I note that there is some sort of catch underneath it which will grant access, what would you recommend in the way of lubricants?
It has always performed well with negs but I've never enjoyed much success with trannies to be honest although it's few years since I have seriously used it. I think I've tried most of the tricks in the book but if you have any definite pointers as to where I might be going wrong then I'm all ears.