Here is some typical debugging output that shows the problem.

In this file, the meanings of the lines are as follows.

 IV id v
    Initialize valuator for slave device id (8 = Synaptics touchpad with
    synaptics driver, 10 = external USB mouse with evdev driver); v = value
    of valuator.  This is called either at initialization or following a
    SlaveSwitch event.
 dv id v -> w
    Change valuator for slave device id, from v to w.  (Note that v may differ
    slightly from w on a preceding line; this is because of the way the
    application handles errors from rounding to the nearest pixel.)
 SS id
    SlaveSwitch to slave device id.

In particular, just after the line "SS 10", note that the valuator is
reported to have value 997, whereas new values of the valuator are
consistent with the valuator being set to 0.

 IV 8 0
 IV 10 362
 dv 10 362 -> 379
 dv 10 378.25 -> 392
 dv 10 392 -> 418
 dv 10 417 -> 449
 dv 10 448.25 -> 476
 dv 10 475.75 -> 503
 dv 10 502 -> 527
 dv 10 527 -> 549
 dv 10 548.25 -> 574
 dv 10 573.25 -> 595
 dv 10 594.5 -> 618
 dv 10 617 -> 647
 dv 10 647 -> 678
 dv 10 677 -> 712
 dv 10 712 -> 761
 dv 10 760.75 -> 813
 dv 10 812 -> 871
 dv 10 870.75 -> 922
 dv 10 922 -> 960
 dv 10 959.5 -> 980
 dv 10 979.5 -> 992
 dv 10 992 -> 997
 SS 8
 IV 8 0
 SS 10
 IV 10 997
 dv 10 997 -> 9
 dv 10 9.5 -> 20
 dv 10 19.5 -> 30
 dv 10 29.5 -> 36
 dv 10 35.75 -> 56
 dv 10 55.75 -> 73
 dv 10 72 -> 79
 dv 10 78.25 -> 82
 dv 10 82 -> 84