Some PNGs not going transparent, but black instead

Weird issue: Some PNGs I upload lose transparency on resizing, going black instead.

For example:

• Original – https://allaboutchris.co.uk/blog/wp-content/uploads/2011/02/answerphoneweb2.png
• Resize – https://allaboutchris.co.uk/blog/wp-content/uploads/2011/02/answerphoneweb2-400×381.png

And yet some images are fine:

• Original – https://allaboutchris.co.uk/blog/wp-content/uploads/2011/02/answerphone.png
• Resized – https://allaboutchris.co.uk/blog/wp-content/uploads/2011/02/answerphone-150×150.png

The difference between the two source images is that the latter is the layered hi res 24-bit PNG from Fireworks, whereas the first one is an exported web optimised 8-bit image: but both still have alpha transparency. Have fiddled with image sizes for that export, makes no difference.

Have checked with our servers, the guys there confirm its not an “old version of GD issue”. Any ideas? Would love to get this sorted!

Bless,
Chris