If you find that there are groups of revisions lumped together in your spreadsheet's 'revision history' that you're unable to access, it might mean that some of the revisions have been "pruned."
Revision pruning is a method by which some of a spreadsheet's edits are merged, or "collapsed" in order to save storage, and allow the user to continue to edit the document. When this happens, the individual collapsed intermediary revisions become inaccessible. However, the net effect of the collapsed edits is preserved in the next version in the revision history. This actually happens very often during normal editing, as it would be inconvenient to have a new full revision created every time you typed a few characters and the document was auto-saved.
There is also a more significant global pruning that can happen as well. When Google spreadsheets senses that the total amount of storage being used by all of your revisions is nearing our limit, it will automatically collapse some of the revisions from throughout your spreadsheet's history to save space.
There is currently no way to retrieve the individual revisions that were pruned. To permanently save a particular version of a spreadsheet, go to the File menu and select Make a copy.