As noted in the comment on verse 17, the band has been traveling for the space of several days on their stored provisions. It is likely that they have either run out, or are very low. Verse 19 indicates that they are suffering "much for the want of food." We are not at all surprised to see Laman and Lemuel complaining for they didn't want to come on this expedition, and find all manner of reasons to complain about their conditions. What is surprising is Lehi. Why is a prophet of the Lord beginning to murmur?
While we cannot really know Lehi's mind, we can speculate as to some of the reason. Lehi knows that the Lord has led them, and knows that they have done all that was asked. Nevertheless, they now find themselves in dire straits, and Lehi is at a loss to solve the problem. Lehi is not a hunter, not the provider of food. Lehi has clearly had no revelation from the Lord to solve the problem. Lehi sees the desperation of their plight, and has no answer for it. His frustration is understandable.
Why should the prophet of the Lord be so left in the dark about the way in which salvation will come? Because it will not come through him. Very clearly the story of the Lehite exodus is one of the development of Nephi as the leader of the group. This is not Lehi's problem to solve, but one which the Lord gives to Nephi.