Hmm, thats not quite supposed to happen. But easy enough to fix.
(Something like) This will be in the next update, but to help you out right now;
Edit the file GFExcelOutput.php
and replace line 117
$this->entries = GFAPI::get_entries($this->form_id, $search_criteria, $sorting);
with:
$total_entries = GFAPI::count_entries($this->form_id, $search_criteria);
$paging = array("offset" => 0, "page_size" => $total_entries);
$this->entries = GFAPI::get_entries($this->form_id, $search_criteria, $sorting, $paging);
Should fix your problem.
GFApi limits the default result to 20 entries (+ 1 header row = 21 rows).
I’ll see if I can make the code a bit prettier. But for now this should work. Sorry for the inconvenience.