Does it always boot into win 10 if you shut it down from win 10 the previous time, as turning fast boot off will fix that:
http://www.tenforums.com/tutorials/4189-fast-startup-turn-off-windows-10-a.html
Win 10 is tricky, it doesn't turn off when you shut it down, it goes into hybred hibernate so that when you start it up, half the programs it needs to load are already in ram. Only time its actually off is during a restart unless you turn off fast boot.
Did you put win 7 on first? http://www.howtogeek.com/197647/how-to-dual-boot-windows-10-with-windows-7-or-8/ according to this it should show it every start up unless you chose a default at some stage.