Subform records change sort order

[Rats]

I have a subform embedded in a mainform which contains loan payment details over a number of months. The record starts with the payment period number ie 1,2,3 etc. The problem is that every now and again the subform changes the display order of the records so that I get 4,5,6,1,2,3.

When I look at the underlying table the order is OK as is the non embedded sub form. I cannot see any reason for the change and it only happens randomly (apparently).

The numbers are not autonumbering but manually input.
Hope someone can help please.

 

[trucktime]

Subform

Don't know what causes it.

The best thing to do is using a query to base the subform (and any other form) on. In the query you can specify the sort order of any field or combination of fields.