Hi Mario,
Open the form in Design View, click the text box in question, and check its Property Sheet > Data tab > Control Source. If this is blank or contains an expression (not a plain field name), the box is 'unbound', anything typed into it never gets written to the table at all. If the ControlSource property setting is an expression, the value displayed is read-only and not saved in the database.

So please ensure it is linked to the correct field in the underlying table or query.
Check the form's Record Source property and confirm that the table or query is updateable and not non editable.
After entering data, try pressing Shift + Enter or move to a different record to force Access to save the record.
Verify that all required fields for the record contain valid data.
If the issue started recently, check whether any macros or VBA code were added to the form's events (such as Before Update, After Update, or On Current).
For reference:
ControlSource Property | Microsoft Support
Set the record source for a report | Microsoft Support
If the issue persists, please let me know some additional information to help investigate further:
- Which version of Microsoft Access you are using.
- Whether the issue occurs on all forms or only one specific form.
- Whether the affected text box is bound to a table field.
- If any error messages appear when you navigate away from the form.
- Do any other fields on that same form save correctly, or does nothing on the form save?
- Does the data disappear only when you move to a different form, or does it also disappear if you just click into another box on the same form?
To ensure I provide the most accurate assistance, I would appreciate any additional information you can share regarding your current situation. If possible, please include screenshots, as they can be very helpful in the investigation.
Thank you for your patience and understanding. Should you have any further questions, please feel free to contact me at any time.
If the answer is helpful, please click 'Yes' and kindly upvote it.
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