A crowded load can hold moisture around the food and restrict the hot-air paths that help its surface dry. More time alone may not reproduce the intended result.
What the observation means
Frozen pieces placed in a deep pile do not receive the same exposure as a single layer. A high-sided tin can behave differently from the shallow tray specified on the packet. The food manufacturer's directions assume a particular arrangement and sometimes a preheated oven. An appliance fault should not be inferred from a load that differs substantially from those conditions.
A useful next step
Follow the packet and oven instructions for quantity, tray and spacing. Note whether the issue occurs only with a heavily crowded load or also with normal portions. Do not move a tray onto the oven floor to obtain stronger heat; use the permitted shelf positions.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.