Data loss in MS SQL server

I use visual Basic and SQL server 7.0 to write a program for data capturing. The program is used by more than 10 operators. When I find a blocking occurs, I kill the blocking process. And it is reported that thousands of records captured are loss. Iam sure there the transaction is COMMIT after every valid transaction. I think only one record can be lost rather than thousand of records.
Why this happen and what is the machanism of SQL server handle the data?