The parents of the two thousand young men had previously committed to supporting their Nephite defenders with food, but they are slow fulfilling part of that promise. While these provisions came to where their sons were, we should expect that they also sent provisions to other Nephite armies. They were as concerned for their sons as any parents would be, but they had committed to the whole of the Nephite nation, not just for the support of their sons.
Even though the Lamanites felt that they were at a stalemate, they were interested in making sure that the Nephites could not get stronger. Thus, when they saw provisions being sent, the did what most armies would do. They attempted to cut off the supply lines.