Why were the disciples not able to do this? (Mk 9:28-9:28)

“When Jesus had entered

The house,

His disciples

Asked him privately.

‘Why could

We not cast it out?’”

 

καὶ εἰσελθόντος αὐτοῦ εἰς οἶκον οἱ μαθηταὶ αὐτοῦ κατ’ ἰδίαν ἐπηρώτων αὐτόν Ὅτι ἡμεῖς οὐκ ἠδυνήθημεν ἐκβαλεῖν αὐτό;

 

There is something similar to this in Matthew, chapter 17:19.  Mark said that Jesus and his disciples entered the house (καὶ εἰσελθόντος αὐτοῦ εἰς οἶκον).  Then the disciples asked or questioned Jesus privately (οἱ μαθηταὶ αὐτοῦ κατ’ ἰδίαν ἐπηρώτων αὐτόν).  They wondered why they were not able to cast out the evil spirit from that boy (Ὅτι ἡμεῖς οὐκ ἠδυνήθημεν ἐκβαλεῖν αὐτό).  The disciples were concerned that they must have lacked something that made it impossible for them to get rid of this evil spirit that was in that boy.