Contact Form 7 – problem sending when config'd with >1 image uploads

I created a new form with a single file upload field (restricted to jpg and 1mb). Works great, messages send, and are received with attached file in place.
Then I amend that same form, adding a second upload field with the same restrictions (I actually added 3 more fields). This time even though it reports that the form has been submitted, the mail is never received.

My form has the following markup:

<p>Upload images:<br />
  Image 1:  [file* image1 limit:1mb filetypes:jpg]  (required)<br />
  Image 2:  [file image2 limit:1mb filetypes:jpg]<br />
  Image 3:  [file image3 limit:1mb filetypes:jpg]<br />
  Image 4:  [file image4 limit:1mb filetypes:jpg]<br />
(note images must be maximum 1MB in size and JPG format. At least 1 image is required or you won't be able to submit your application)</p>

and my e-mail Attachments field has: [image1][image2][image3][image4]
(I tried it using different variable naming conventions also to no joy)

If I leave the form alone (with 4 file upload fields), but amend the e-mail attachments value so that it only has [image1], then I can successfully send a message with a single attachment. As soon as I amend the e-mail attachments field, so that it reads: [image1][image2][image3][image4], the messages are never received, regardless how many files are attached, despite the fact it reports the form having successfully submitted.

Anyone got any ideas?

Contact Form 7 Version is 3.3.3 (also have contact form 7 datepicker plugin installed)

Thanks, Paul A