This actually involves whether your practice method is correct! In fact, whether it is a fist or a straight fist, it is best to drive the body and attack the sandbag with the waist and hip bones. Be flexible in punching, don't hit your arm straight, leave some strength! If possible, it is best to cooperate with footwork, so that the rhythm will come out! It is advisable to warm up 15-20 minutes before punching sandbags. Secondly, practice should be kept from slow to fast, such a process! You can try the practice video of punching sandbags. Pay attention to the action must be standardized. Need to practice step by step!
Arm exercises?
In fact, in addition to equipment, multi-body pose and push-ups are good exercise methods!
As for your other questions?
There are problems with the way of hitting, whether there is sufficient warm-up and too hard hitting, paying too much attention to the form of appearance and ignoring the connotation of technology!
Suggestion:
Practice learning from the technical stage, and usually pay attention to the practice of overall strength and endurance!