Are you sure you're using the correct heads for the type of cabling that you're using? Solid core wire requires different connectors than stranded wire does. The crimper shouldn't have any great impact on how well the connection is made (unless you're unable to apply enough force, but I'm guessing that grip strength isn't a problem for a rock climber).

Some new heads have dual-mode conductors so that they work on either solid or stranded wire, but it's easily possible that you got one of the ``older'' styles.