my cat doesn't purr even though she seems happy...

she'll jump on my lap, melt onto my hand while im giving head scratches, but she doesn't purr during these moments, even though she usued to purr when she was small(er), and loudly too. i can hear a little itty bitty purr if i put my ear right on her chest, so it might not be lack of trying.

pretty sure she's an adult by now, even though she is a lil smol compared to her older brother. it's been like this for some time, and I'm hoping it's not some kind of harmful condition.