Queries only working with the first 299 rows

Hello everyone, I am having a bit of an issue with SQL Server 2008, I am trying to know why my queries are not working correctly.

They only affect the first 299 rows or so, even if I do a "SELECT * FROM tableName" it returns an incomplete result, if I want to update and set one field to a single value, same thing happens.

Do you have any ideas of why this could be?

Thanks in advance;

Kindly, E