Smeg DF612 Se – Error code 6 – Replaced drain pump 3 times!

My smeg has developed error 6 – which is “the appliance is not able to pump out water from the tank”. I have bought and replaced the drain pump 3 times and each time it is still not clearing the water out through the drain hose. I have checked and cleaned all the filters and removed the hose and replaced and checked the drain sump throughly for any blockages. It will run through a cycle and when it trys to drain I get the error and the bottom is full of water. The only way I can get it to actually drain is to lower the drain hose to almost ground level and attach a really long hose through the kitchen and out the door, which is not really ideal! I am at a loss of what the issue as, as I have tried 3 different drain pumps and I am still getting this issue. Is there another possible part failure/fault that is causing the machine not to pump the water out? Any help would be appreciated. thank you