LOCK on concurrent MODIFY, but not the same record

Hello,

I am trying to avoid Lock between users when they are modifying data. I have some locks even if modifications are not on the same set of records.
I made a test with 2 users: one is modifying customers whose name is beginning with "a" (filter = 'a*'), the other is modifying customers whose name is beginning with "b" (filter = 'b*').
I have tried several ways in my code : with FIND(FALSE), FIND(TRUE), FIND('-').... The result is always the same: one user is locked by the other one.

Do you know if there is a way to solve this problem.

My version is NAV 2009 R2 Classic, with SLQ database.

Tanks in advance.