Do you have an in-line filter on the Agilent? I use one constantly on mine. If it becomes slightly plugged it causes peaks to split. You can tell if it's plugging by taking the column off and checking the back pressure of the system without a column in line. There is also a Teflon filter in the bypass valve that can cause you all kinds of grief when it starts to get plugged. At 0.3 mL/minute your back pressure without a column should be on the order of 2-4 bar.

We have never been able to routinely inject 20 uL using 0.3 mL/min of flow on the 1100 - it just takes too long to all get to the column and ends up causing band spreading. When I run narrow bore (2.1 mm) columns I always drop my injection volume down to 5 or 10 uL. Also, if you run narrow bore columns you need to make sure you have a small volume UV cell - the standard cell will cause you grief with low flows.