FThrow to BF is only guaranteed up to a certain percentage, less so if they DI up and towards. (Unless you're fighting Roy, which then seems to work forever.)
Yes, you should buffer BF as soon as you see the FThrow animation start.
Here are situational examples:
Every character except Jigglypuff, 0-10%, no DI/DI away: FThrow, buffer BF and keep pressing B to make the kick come out early. Guaranteed every time.
Almost every character, 0%, up/toward DI: FThrow, buffer BF and then immediately hold back on the stick and keep pressing B to make the kick come out early. Sometimes you'll have to buffer a short hop before BF to get more verticality. But the key is holding it back with the control stick and making the kick come out early.
Most characters, 30%, no DI/DI away: FThrow, buffer BF and then hold the direction toward your opponent and keep pressing B.
Most characters, 30%, up/toward DI: You might be able to do a short or full hop held back shortened BF, but you should probably just go for aerials.
Most characters, 30-50%, no DI (or sometimes DI away): FThrow, buffer short hop then perform BF at a 45degree angle (
,
) and then immediately the full on direction (
,
) and keep spamming B. The jump gets you higher, the directional influence makes you go farther, and spamming B makes the kick come out earlier. They might not have the time to airdodge. If they do manage an airdodge, you could try doing a rising double jump aerial to hit them anyway.
High level players are probably in the habit of DIing away because eating true combos at early percentages is probably not as bad as eating true combos at kill percentages.
Sheik is great because she has options to easily put people in different 50/50 situations almost regardless of % ranges. If your opponents are constantly going for up/in DI after FThrow at early percentages and you see that you're just not getting conversions with BF, switch up your game. They're in the habit of doing this and they're leaving themselves open for full hop double Fairs, Nair, and Uair.
If you find them airdodging your fast out of throw combos, this is a good sign. They're spamming/buffering it. It means you can snag earlier kills by doing FThrow, dash in, charge Usmash between 70% and 90% (they airdodge right above you and then slowly float right into your charged sweet spot Usmash); or DThrow into FH/double jump Vanish between 100% and 130% or so. If you get past 130%, you can again Fthrow, full hop, wait for air dodge, and then BF. Encourage them to airdodge, it leads to their deaths.
