Help with cooling down house at night during cool temps House is about 1900 sq ft., moved in about 3 months ago live in the Midwest.  We've been having cooler temps at night and I want to take advantage of it.  My goal is to get an attic fan in eventually but money wise right now I can't afford to put one in.  

Sorry for the long winded intro onto my question.  
 Example it's 78 in the house when I get home from work (no ac on today with only 1 window open) Its 65 outside right now windows have been open with fans on for 4 hours and it's currently 74 inside. If the low is say 60 it won't get below 70 all night 

We have 2 box fans that I put into windows and turn on the central furnace fan to circulate air but it still takes hours to get the temp to go down a couple degrees when it's easily 10 degrees cooler outside then in.  What am I missing?  