Should the answer of an istikhara always be followed and is it haraam not to follow it?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
Praying Salatul istikhara is a sunnah. The Prophet (pbuh) would teach the companions salatul istikhara just like the Quran was taught.
Salatul istikhara is a dua which is made when we have already investigated a matter, sought consultation and yet are still unsure, we ask Allah (swt) to guide us to an answer which will be good for us. The crucial point is that we are turning to Allah (swt) here. It is not necessarily that we will get a dream in response, or that the dream will be clear. It may be just a feeling, or that certain things just fall into place as Allah (swt) willed. The fact that you feel an inclination to not follow what you think you should may also be a sign for what you should do. If you still feel unsure the istikhara can be repeated up to seven times.
Imam An-Nawawi says that “after performing the istikharah, a person must do what he is wholeheartedly inclined to do and feels good about doing and should not insist on doing what he had desired to do before making the istikharah. And if his feelings change, he should leave what he had intended to do, otherwise, he is not completely leaving the choice to Allah, and would not be honest in seeking aid from Allah’s power and knowledge. Sincerity in seeking Allah’s choice, means that one should completely leave what he himself had desired or determined.” 
Whatever decision you make and whatever you feel inclined towards, have faith that it is all by the Will of Allah (swt) and that there is goodness in it, even if you cannot perceive it at the time.
For more information on istikhara, see the following link:
Only Allah knows best
Written by Alimah Nasima Umm Hamza
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham