Ok, maybe. [Trying to explain how-it-could-work to myself] If touch vibration used to be weaker than other vibrations, then it is possible that a piece of something stops touch vibration, while other vibrations get through...

Recommended solution (for N900, at least) is:

switch on the vibrator, so that it would vibrate without stopping
dbus-send --print-reply --system --dest=com.nokia.mce /com/nokia/mce/request com.nokia.mce.request.req_vibrator_pattern_activat e string:'PatternIncomingCall'

Wait for some time while the vibrator is working; whilst it is working, hit the phone to help the motor to un-stick the piece of something

switch off the vibration (so that it would go back to vibrate-on-touch or whatever its usual behaviour is)
dbus-send --print-reply --system --dest=com.nokia.mce /com/nokia/mce/request com.nokia.mce.request.req_vibrator_pattern_deactiv ate string:'PatternIncomingCall'

Thank you. Best wishes.
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Per aspera ad astra...