Summoned fireballs frequently enter a state where they become invisible and reappear but also become non-interactive, so you cannot punch them. 

Reproducing is not 100% reliable however:

To Reproduce:

• /Summon Fireball ~ ~ ~ {direction:[0.0,0.0,0.0]}
• Move around the fireball to observe whether the bug is occurring (This will be evident as it will become invisible as you move around)
• Otherwise summon a new fireball
• Otherwise create a new world and summon a new fireball