It's happened multiple times???
After the first time it happened you should have been very suspicious. Withholding sex is a massive red flag. It's usually caused by low attraction, but could also be a game to manipulate you. Either way, it's bad news.
So you gave her the benefit of the doubt - fair enough, but it happened again. This is when you should have realized something was seriously wrong. The second time it happened you should have backed off and withheld attention.
Seriously, I know this may be an oversimplification, but generally as a comparison of male-female dynamics: SEX = ATTENTION. Men value sex very highly while women value attention very highly. So when she gives less sex, you give less attention. It's like men and women's currencies.
At the moment it's like Sodbuster said - she's getting her needs met (attention) while you're not getting your needs met (sex). You're falling into the friendzone role of cuddle b!tch. Sort it out!
If she won't put out, stop hanging around her looking for scraps. Right now she has NO FEAR YOU WILL WALK AWAY. She's got too comfortable. Shake it up a bit. Spark attraction. Get her wet for you again. Turn her on.
COMFORT =/= ATTRACTION.
Spin more plates.