You don't say how much experience you have of HF but here goes.
Do you have a chart which lists range from the station you are calling against time of day to assess the best frequency? If the sun is high at the midpoint between you and station you want to contact then go for a HIGHer frequency, if the sun is LOWer go for a LOW frequency.
That said if you get no joy just try any frequency for really long range (several thousands of miles). I recall going down to Mombasa one night and decided to patch Ops via Stockholm to advise progress. Tried all the theoretical freqs with no success. So decided to try them all! Got Stockholm loud and clear on a very low freq (was it bouncing the long way round the world?).
Even HF SSB can be a bit hit and miss (you should have tried it when all we had was AM) but perseverence pays off in the end usually.
Good Luck!