DD has been terrible with day sleeps for the last month or so. The last week she has had only one day sleep more often than not, and I too feel she is way to young to only be having one sleep a day (she has been in bed by 6/6.30 though as by then she has been awake for 5-6 hours!) but nothing I do will get her to go to sleep.
If she hasn't gone to sleep in an hour I usually get her up, it's not worth me stressing over her not going to sleep and her just having a good old time rolling around in her cot. So I don't have any real advice for you as I'm going through the same thing, just persevere and hopefully she will realise that she does need to sleep!