Help with DSLR flats please!

[Black Knight]

For some reason, the process that I have been following to capture flats up until now has suddenly stopped working. I am now left with vignetting in my image, which I had hitherto managed to eliminate with flats. I haven't changed my capture process at all, so I'm a bit perplexed.

The process that I've been using up until now is:

• cover the scope with a white t shirt
  
• point the scope at my laptop screen running Notepad maximised
  
• set the Canon to AV
  
• use APT to automate the capture of flats (20, 40, 60 or whatever)
  
• the histogram shows about 30-50% luminance
  
• For the flat I have attached, the mean value (according to Nebulosity), is 11073 - which seems low to me
  
• use DSS to integrate the files
  

As I say, this process has been working fine up until now, but now for some reason it doesn't!

Does anyone know what could be going on, or can point out a flaw in my process? I'd be much obliged . I have attached an image I took last night of M13 integrated with the offending master flat (from 40 flats taken), and a jpg of the master flat used.

Thanks,

[Black Knight]

Gah! Sorry, wrong piccie! Heres my M13 integrated with the above master flat. 'Orrible, innit?