I would hazard a guess there's a card fault and the camera/s can then not associate a file tag with the image connected with it so thinks there's no image there to display on some of them.
As Dave said, don't delete/format cards on the PC and do it in-camera. When either happens, the images are still there but the camera can no longer read them so thinks/shows the card is empty and away you go again taking photos which are now over-writing images that are still actually there, not erased.
I've noticed with Canon D-slrs, don't put a card from another Canon D-slr into one of yours--say a friend loans you a spare card from his. The file numbering will go up the spout when you go back to using one of your own cards----to find the image number is now continuing from your friends camera and won't go back to your numbering if his card's images were lower than yours.
My Nikon cameras won't show images on a card that was taken with a Canon. My Canon d-slrs will show the images on a card that came from a Nikon. If you use the image number as a guide to shutter actuations, I find it best to keep the same card with the same camera and don't swap them about. My Nikons, bar the D1, will show actuations anyway on the metadata as does my Canon 1Ds MKII. My other Canons encode the actuations and will not readily appear as such in the metadata to easily read.
All my cards are San Disk, top end range. When shooting a wedding, with dual card slots on the D3, each image is being written to two cards just in case one of the cards does fail; the 2nd card has the same images as a sort of back up, as well as spare cameras.Edited by: Paul Hilton at:12th June 2012 11:59