Check the channel count at every network intersection. If any of them is even close to max load, investigate to make sure you didn't miss any "non-critical" devices that might have gone offline ages ago. Level Emitters also use a channel, for example.
Check if the energy cells actually drained. If you input too much power into the network so they fill up near-instantly as you approach, use a TE cell, EIO capacitor or similar device to throttle your input. Make sure you input only a little more power than your system needs, then play normally until the buses fail. If the local energy cells are empty/almost empty you're running out of power.
I assume you have already checked for "intentional" breaking points or cross-mod problems. In case you haven't, check for toggle buses, quartz fibres, orphaned P2P connections and similar AE2 mechanics. Check for accidentally placed filters and cards too (block type whitelist, inverter card, missing crafting card) Anything that moves blocks can also be an issue, does your design include (semi-)automatic spatial IO, frames, Teleposers or other ways of changing the system configuration without manual input?
Or maybe the buses themselves aren't the problem at all. What are you actually trying to do, and which buses aren't working?