For 10, you must have just counted white keys wrong...
For 11, yeah, you're approach is wrong. You can't just assume that if you count white keys you'll end up with either a major or perfect interval. That ONLY works if you start on C and go UP. It doesn't work if you're going down or if you start on a note other than C. You need to actually count half steps in addition to counting white keys. Counting half steps tells you what note it is (i.e. on the keyboard) and counting white keys tells you which enharmonic it is (i.e. is it B#, C, or Dbb?). You need to do BOTH in order to get the correct answer.
Remember that, when counting white keys, you count the starting note, but when counting half steps, you don't. You seem to be doing fine remembering this, but it's good to reinforce since it can be kinda confusing.
So, I'll walk you through 10 and 11, and then see if you can figure out 12-15 on your own.
10. We want to count up a M7 from F2. Counting up 7 white keys from F2, we go F2, G2, A2, B2, C3, D3, E3. So the answer must be some kind of an E (either Ebb, Eb, E, E#, or Ex). The interval M7 is 11 half steps, so counting up 11 half steps from F2, we go F#2, G2, G#2, A2, A#2, B2, C3, C#3, D3, D#3, E3. So the answer is E3.
11. We want to go down a m3 from C3. Counting down 3 white keys from C3, we go C3, B2, A2, so the answer is some kind of an A. The interval m3 is 3 half steps, so counting down 4 half steps from C3, we go B2, Bb2, A2. So the answer is A2.
And I just realized I forgot to give an octave # for question 15. Lemme give you one:
15. From ___ to Bbb3 is d3.