Why did the disciples doubt Jesus could feed the multitude the 2nd time?

Matthew 12:

1 At that time Jesus went through the grainfields on the Sabbath. His disciples were hungry and began to pick the heads of grain and eat them.

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On this occasion, Jesus did not multiply food to feed his disciples (Mark 2:23)

Sometime later, Jesus fed the 5000 in John 6:

5Lifting up his eyes, then, and seeing that a large crowd was coming toward him, Jesus said to Philip, “Where are we to buy bread, so that these people may eat?” 6He said this to test him, for he himself knew what he would do. 7Philip answered him, “Two hundred denarii worth of bread would not be enough for each of them to get a little.”

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200 denarii were half a year of wagers for a laborer. Philip reacted with puzzlement. He didn't know what to do.

8One of his disciples, Andrew, Simon Peter’s brother, said to him, 9“There is a boy here who has five barley loaves and two fish, but what are they for so many?”

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Jesus proceeded to multiply food based on this meager amount. It was a miracle (Mark 6:43).

Sometime later, a similar situation arose in Mark 8:

1In those days, when again a great crowd had gathered, and they had nothing to eat, he called his disciples to him and said to them, 2“I have compassion on the crowd, because they have been with me now three days and have nothing to eat. 3And if I send them away hungry to their homes, they will faint on the way. And some of them have come from far away.” 4And his disciples answered him, “How can one feed these people with bread here in this desolate place?”

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Did the disciples react by doubting Jesus could do it again?

I don't think so. They reacted with a question, not a statement. They didn't know what Jesus was going to do and they did not want to presume. The disciples learned to defer to Jesus's judgment.

Sure enough, Jesus proceeded to multiply the loaves and fish for the 2nd time.