The solution to the problem that the internal temperature of commercial yogurt machine can't reach is as follows:

1. Check whether the power plug and power cord are connected properly. If it is loose, it needs to be plugged in again.

2. Check whether the thermostat of yogurt machine is set correctly and works normally. You can try to turn the thermostat up or down to see if it can reach the expected temperature.

3. Check whether there are foreign bodies or dirt inside the yogurt machine, and clean it if necessary.