There is a big difference in reading and sightreading. When you're "reading" you have the possibility to slow things down or to put it together piece by piece. "Sightreading" however requires that you play in the right tempo with all rhythms from the start. In this regard "reading" is pretty simple and most people are able to learn this quickly. But if you really want to learn to sightread you have to stop slowing things down and this makes it much harder.
For my students I recorded some sightreading videos like this here:
The aim is to play along to the video without stopping. This is the real-life sightreading situation. Can you do it?
To practice that the routine would be something like
a) play along with the video
b) don't stop if you make an mistake, play through the whole piece in time
c) you'll probably make a lot of mistakes on the first run-through
d) play it again and try to make less mistakes
e) reapeat this procedure until you have it 100% right
Some people don't like to practice that way, because there're afraid of making mistakes, which is inevitable with this method. But making mistakes is the key for learning here and it really helps. By eliminating the mistakes one by one you learn to transform the written music onto your fretboard in time.