External tank not filling

I recently encountered the same issue. First off, with the external tank installed is there an icon on the display panel indicating the tank is installed?

No? Then the two sensors are not picking up the magnets in the the "arms" on the back of the tank. It's a poor design for sure, on my unit the tank does not sit flush enough to the back of the AC, if I held the tank firmly to the back the icon would come on. I just used a bit of cardboard at the bottom of the support tray to press the tank firmly to the back of the AC.

Theory is as the external tank fills up, water will run into the "arms" and raise the magnets tripping the switch which tells the unit it needs emptying. But the magnets in the arms are not strong enough if they are not close enough to the sensors. So my cardboard solution lasted only one tank.

I ended up getting some flat neodymium magnets (that I scavenged out of a hard drive), and taped to the sensor that had the gap. I wouldn't recommend doing this to both sensors since it would eventually overfill, only one sensor needs to "trip" for the compressor to turn off.

If you answered yes to the first question (the bucket icon is there), then it's most likely the pump not working on the internal reservoir, or something related to that. Can't help you there...

the external tank must be lower than the internal tank, water does not flow uphill.

1.) Are you sure that you have the external tank connector properly pushed into place - you'll feel or hear a click ("this connection is your sensor) ". If a proper connection is not made , it will block the water flow to the external tank and divert it back to the internal tank.
2.) Are you getting water in the INTERNAL tank? If not, check under the A/C for water (if it's on a carpet, you might not see it and will have to feel to see if the carpet is wet). There is an internal hose that leads to the external tank, if this hose leaks it will send the message that the External tank is connected , when in fact although it's connected, the water is not getting there. Therefore water will continue to try to flow to the external tank. However; as the hose has a leak (is broken - "not uncommon"), the water will not reach the external tank, and will drip onto the base (floor panel) of the A/C; and after time will find its way through an opening (usually around one of the wheels), to the floor; and, as I previously stated; you won't notice if it's on carpet. Hope this helps
K.S.
NOTE: The QUICK CONNECTOR Danby has installed on these units , (to my mind) is not user friendly and as a result it creates a problem for the average home user to connect. This results in undue stress on the above referred to internal hose; which, can lead to breakage of the hose or leaking connections.