Milk supply is decreasing....

[vanimal]

I am BFing my 4 month old DD, and I express her 10.30pm feed. I give her this from a bottle then put her to my breast (I can't give it all from the breast as she falls asleep immediately and won't feed).

I can usually express around 5oz at this time (just about), but have been finding that my supply is dropping - last night I barely managed 3oz.

I can't really express at other times of the day as I am invariably busy with my toddler.

I don't know why this is, is there anything I can do to pep up the supply?

when you bf at other times, can you hold a bottle to the opposite breast and collect that way - if you leak milk from the other side when feeding? This is a great way of catching milk whilst dd is feeding.
Or (b), wake up a bit earlier than dd and toddler and express then when you have lots of milk. I wouldn't have thought that by expressing just once at 10:30, your supply would decrease.
Make sure that the pump is sealed properly around the breast as any lack in suction will greatly reduce amount.

You could always try feeding DD half an hour earlier and see if she takes bf instead of expressing; she might not fall asleep then?