how old are they? Your see them breed in the morning/afternoon a lot of the time. If she just laid her first "clutch" as a dud her next one might be fertile.
If those are 2 year old geckos I'm going with you have 2 females.
The other reason I feel like that second photo could possibly be female is that the base of the tail looks as if there is one lump in the middle vs. having 2 lumps(hemipenes). You said also you've gotten something like 15 dud eggs from the female. How long of a period does this include(like over a couple months or a year?)?
Yes I've had these guys since October 24 2012. It's been awhile. I'm leaning towards the current terrarium size not suitable for my geckos which is why I believe they're not breeding. Any advice would help. On the side note: According to my vet that my geckos are very healthy and well taken care of. They're a little beat up because they fight from time to time but everything else looks good. So I'm dong something right lol
Can you get some more photos of their vents? A few different angles of each.
How many dud eggs have you gotten and do you know the time frame from when you got the first couple to the most recent?