She may be getting too much sleep at her first two naps and therefore unable to get in anymore in the catnap. But at her age, you risk her getting overtired and having nightwakings without the catnap.
I'd suggest not letting her sleep more than 1.5 hours at the first time naps so she's tired enough for at least a 30 minute catnap. If that still doesn't work, you can limit one nap to 1.25 hours. Also, the catnap is considered a "freebie" so if she won't take it in her crib you can allow this one to be in a swing, stroller, car, etc as it's going to go away soon anyway.