All tests can have a different sensitivity so its hard to compair the two. Usuallya a line is a line as long as it came up within the time limit and it has colour to it and can be seen at arms length.
I would try another test brand altogether though especially if you have had false + on first response and fortel extra is known for false+. I used Discover and got my BFP on 10 DPO as you can test 7days before a missed period. Good luck and I hope it is a BFP for you.