Data displayed in subform reverts to first record of data.

I have a small database for saving customer records. Normally it works fine.
When I enter new data for customer in the sub form and save it, the subform
displays the current data. However occasionally the data displayed after the
"save" is the first record for that customer. I then have to move through
the database to find the correct sub form record. I can correct this problem
by "compacting and repairing the database" - I have to do this every month or
so. Can anyone please tell me why this is happening and what I can do about
it? Thanks in advance.

Peter