Check your lower limit setting, you should only have a millimeter or so of clearance between the chain and derailleur cage in the small front/big rear combination. If you leave too much clearance you may run out of travel at the high end.
And try setting the high limit by tugging on the cable instead of pushing the derailleur; if there is play in the pivots you may need a different limit setting when using the wire to move the derailleur than when pushing it.