Data entry is a mode that opens the form to a single new record to allow you to do exactly what the name infers. It is not 'editing'. If you cannot edit any of the records when they are all displayed, you're probably getting a warning or error message that you're not sharing here. Open the query that the form is based on and see if you can edit anything in the query datasheet view. If you cannot, the problem is with the query - some are not editable because of their joins, some because aggregate functions are used in it. If you can, some other form property is not set correctly, such as 'Allow Edits'.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.