TBH, it's pretty cruel leaving them locked up til the afternoon. Let them out first thing in the morning. They naturally go to lay where they are used to laying which by the sounds of it is in the nest boxes in the coop. Hens like to lay where it's draught -free, warm and dark so the likelihood that they'll lay in the garden is slim to none.
The more daylight, access to grass, grubs,feed and water will offer your hens the best environment keeping their health at a premium thus maximising egg production.
I currently have 10 hens all of whom are let out first thing and all lay in the nest boxes in the coop each day. If you need more info on looking after chickens, I'd recommend this site, http://www.henkeeping.co.uk