The symptoms you describe suggest a corrupted index as the cause of the
problem.

When you select a field in table design view, the properties (lower pane)
allow you to specify whether the field is indexed. You can also view/edit
indexes in the Index dialog (View menu in table design). Additionally,
Access creates hidden indexes when you create relationships with referential
integrity enforced.

Access then uses these indexes to select and sort records. Which index(es)
it uses depends on your criteria and ORDER BY clauses. If an index is
corrupt, it may not contain all the records, so when your sort/filter
includes this field, the number of records will be fewer than the whole,
because the index is incomplete. "Fewer" can be just one unindexed record,
or it can be only a small fraction of the records if the index has been
truncated - usually because of an interrupted disk write (i.e. the computer
crashed, network failed, power failed, or user switched off while a write
was incomplete.)

Conceptually, think of indexes as being like the old library card indexes
that can find a book by author. When a new book is added (or the author
changed), the index "card" has to be created and inserted. If this process
is interrupted, the "last" card in the index might be an author starting
with B. Since this is marked as the last index record, the index insists
there are no more records after it reaches the Bs. (I actually saw that
once: no customers after B, due to a corrupted index, so the database
returned 600 instead of 13000 customers in some views.)

Most times, you can rebuild the indexes simplying by choosing Tools |
Database Utilities | Repair. In severe cases, it may be a matter of
carefully deleting the relations and indexes, and then exporting the view
that returns all the records to a new database, from where the application
can be rebuilt.

This kind of corruption is almost non-existent in a stable environment where
the power, network, hardware, and users are working reliably, the objects in
the database (tables, forms, reports, modules, ...) are not being modified,
the database is split (if multiuser), and service packs are applied
consistently.