Missing data from old form versions
Because the current (latest) version of a form governs how data is formatted and exported, it's easy to end up in a situation where data collected with an older version of a form doesn't display in the Data Explorer and doesn't appear in exports. The data exists – it hasn't been lost! – but it's not showing up when you look at or export a submission, so it certainly seems lost.
Take a simple example: you delete a field and deploy an updated version of the form. If you had collected data with an older version of the form, then you might have data for that field. However, since the latest version of the form doesn't include that field any more, when you review an older submission in the Data Explorer or export your data, the field doesn't appear. SurveyCTO has data for that field, but there's no field in which to show it when you review the submission, and there's no column in which to export it when you export your data. So you don't see that data.
Take another example: you rename a field. From SurveyCTO's standpoint, a renamed field is really just one field that was deleted and another one that got added. So if the current form collects data for a field named "age", then that's the field that shows up when you review submissions and the column that appears in your export; if you have data for an earlier field named "childage", but no such field now exists in the form, then that data won't show up anywhere.
A far more subtle case concerns changes to the groups within which fields are located: SurveyCTO keeps track of fields a little like computers keep track of files, using not only the field name, but also the groups within which the field resides. So a field named "age" in groups named "demographics" and "youngestchild" is tracked, internally, as /demographics/youngestchild/age. In fact, that field is treated as a different field from /demographics/oldestchild/age, even though both fields are named "age". If you add, remove, or rename groups, it implicitly renames all fields within those groups, which has the effect of making it like some fields have gone away and some new ones have been added. This can then have the net effect that data from older forms disappears from the Data Explorer and from data exports.
Please see Updating an existing form for a full discussion of how to safely update existing forms, and for more on the implications of (and remedies for) different kinds of changes to your form. If you need more help, you can post a message or open a request in the Support Center.