The area around Manti was a connection between Lamanite and Nephite lands, and that meant that when the Lamanites controlled it, they could easily augment both their numbers and their provisions from the Lamanite homeland. The Nephites should have had that same advantage, but some circumstance that Helaman did not know at the time was keeping his army from the same advantage. Thus, even though he was in a siege-like position, it was not an effective siege because they could not cut off the city from supplies.
On the other hand, the Nephites were suffering from the kinds of problems a sieged city might have undergone, because they were not receiving reinforcements and supplies. In verse 7 we learn that the situation was becoming dire.