Power Reserve Of Eta Question

[simmons3d]

I recently purchased a ladies datejust with the ETA 2671-2 for the girlfriend. Unless its wound up manually the watch won't run for more than about 10 hours off the wrist - even if worn most of the previous day. Compared to my 2836 Sub its somewhat dissapointing.

Is it something to do with its tiny size? Does it need a service?

Any suggestions are appreciated!