They don’t want to pay benefits so they don’t give people consistent full time schedule. They might give it for a few weeks like they did with you. It’s common to hire bunch of part time people to work a few hours rather than a few full time employees. Also most part time positions cannot guarantee number of hours. It fluctuates.

If you want job with benefits, you need to only accept full time job with benefits. I don’t recommend taking on part time job with hopes for more hours and benefits. It’s typically too unstable.