Why are things not saving to my SD card?

Minor factors like full cache, low internal storage capacity, corrupted SD card could be the reason why your camera is not saving on your SD card. If you are using an android phone like Tecno, then it could be that your phone has developed certain issues that could cause this.

I think before buying a phone, it is better to check with SD card card

You are right. You should check while SSD is on the phone.