Bobbin case in my singer 2723 pop out everything i sew.

SOURCE: singer solid state-bobbin part

Check your tension disks, with your presser foot up, and the tension dial on zero. Sometimes a piece of cotton breaks off between the tension disks and then prevents the disks to work properly. Also make sure when you thread the machine, the thread does go through the disks and over the take up lever. After you threaded the machine, before putting the thread through the eye of the needle, lower the presser foot and while pulling on the thread with your left hand, increase the tension with your right hand. Do it slowly and you should feel in difference in tension. If this is the case, your machine should work fine. Also use a ball-point needle on t shirt material. Good luck!

SOURCE: bobbin case retainer popping out on Singer Touch & Sew 625

Take the bobbin case out. Then when you go to put it in, realize there are 2 ways to get it. You have found one way. The other way is first look in the the slot with a flashlight before you insert it. There is a flat piece of metal the bracket needs to be slid under as it is inserted. This acts like a spring making it tight

SOURCE: bobbin case pops out wehen sewing.

The case probably has a scratch or a burr on it. Take it out of the machine and examine it for burrs or scratches.If you find some they can be removed with an emeryboard and then buffed with an SOS pad to make it very smooth.
When you put it back in the machine, thread the machine and sew by turning the wheel a few times before you actually begin sewing for real.
sewman7

SOURCE: hand held singer sew quick

Remove the plastic pin holding the bobbin (thread source) - careful to not lose the spring.

You may need a pair of pliers to get it loose the first time.

Press down on the edge of the bobbin so the pliers can grip the pin - lay a piece of material over the pin so the plies do not gouge the plastic - twist and pull.