Home not cooling at night
Here's what may be happening...

At night, heat stored in the roof and walls due to solar gain soaks into the home's interior. This is balanced to some extent by cooling from outside air flowing through the home. Three outcomes can result:


The home cools somewhat if the inside/outside air temperature differential is substantial, and if there's a breeze to move air at a significant rate. 
The home stays at the same temperature if there's not enough breeze and/or the temperature differential is small. Heat soak and cooling are essentially balanced.
The home actually warms, especially on the upper levels of a multi-level home, due to heat soak. This occurs if there's little or no breeze and the temperature differential is small. 


The change you've noticed is probably related to lighter overnight winds and slightly reduced differential. If you don't have a differential of at least 10 degrees, and the wind is calm, you'll probably want to bite the bullet and keep the a/c on. Run circulating fans and/or the furnace fan to keep all areas of the home comfortable. 

If you do open the home up in questionable conditions, run box or dedicated window fans to exhaust the home overnight. Run fans inward in sleeping rooms for maximum comfort.