Now that you have the window secured in the opening, check to see how well the window is aligned when it closes. Sometimes the gap is uneven and this can be corrected with the screws.
First determine which direction the window needs to move. In this case the top needs to go right and the bottom needs to go left so you first loosen the screws on the top and the bottom that are opposite to the direction you want to move the window. In other words, loosen the top left one and the bottom right one. Now tighten up the screws in the corners where you want the window to move towards.
Keep adjusting the screws in each corner while checking the gap. Each screw should be adjusted a little each time so that you take up the slack evenly, top to bottom, and so that the whole window isn't pulled off center. Be careful not to pull too hard on any one screw because you could damage the window. When the gap is even (or as close as you can get it) from top to bottom, snug up the loose screws again being careful not to draw the window out of square.