AliExpress complaint: not received goods, no refund

It happened twice. I order something on AliExpress, I receive an email stated thet there is signed for the goods. But I have not received anything! When I ask a refund they say I have signed for the goods…
I know this is a very complex problem, because anyone can say that they have not received the goods. I believe the courier is the problem. But AliExpress is accountable that I receive the goods in a good condition. I think AliExpress should contact the courier and sort it out with them. I have payed for items I never received!
I order for many years goods at AliExpress and I have never had this problem. Just recently this occured twice!
AliExpress should also look at my history and that I always pay for the goods ant take advantage of anything. But now I become hesitant to order again…
Such a pity, I really liked what they offered.