Welcome to the Forum and ImageJ in general.
So looking at your image - you can see that your sample varies across the background… you have a brighter background on the left side which diminishes as you move towards the right side. That is why a simple threshold won’t really work across the entire image.
You’ll have to do some pre-processing to deal with that background variation. Take a look at these older forum posts - you might find helpful:
Here are some other helpful links that you can use to get you started in Segmenting those cells (though know that things won’t be ‘perfect’ because of this background issue - just clean it up as much as you can):