ok, you're right glabal means for all peoples, local for each. You should keep a variable local or global through out the game though... Your strength should look a little like this, yes?
define variable <strength>
type numeric
scope local
value <5>
end define
Now in the multiplayer game, quest turns this into an array, with the index buing the userid, %strength[userid]%...
Double check you've defined it correctly... also if you don't need it displayed at the side, I sugeest not havinc it as a status variable but a normal array, which is defined by putting 'set numeric <strength[10]; 5>' in the start script, this'll set an array called sterngth with indexs of 0 to 10 for you, each set to 5.
If this doesn't help, send me your game and I can check for a possible cause. Hope this helps.