In recent weeks I have heard several writers read out short stories they have written. Interestingly most of these stories started really well and drew you into the action and characterisation quite quickly. They then tailed off and with the concluding part of the story a disappointment.
It is possible these writers began their stories with the beginning and opening very clear in their mind and then, as they wrote, thought up the conclusion.
Some writers do write this way and see where their stories and characters take them. I know one detective writer who does not know whodunit until their story is quite advanced. However if you write this way, once you have written your story you do need to consider your conclusion carefully.
Does it satisfy the reader?
Is there a good twist?
Does it end in the strongest possible way?
Does your story end with a 'bang,' ie deliver the conclusion and then finish rather than peter out.
The beginning of any story is very important as you need to draw the reader in. But the ending also has to be strong, particularly if you are hoping to interest an editor with your work.
If you write fiction, do pay attention to your ending and make sure it is strong and satisfying. Just as writers like to give thought to beginnings, this is equally true of endings - and besides, a strong ending can always increase the chance of acceptance.