Low Poly Pathfinder Issues

What I do, I use the paint bucket (Live Paint tool)

Low Polygon illustration using a face

Place the image in Illustrator

Right click over the document and select show grid

From the Grid preferences, I set up the grid to see small squares like the following image.

Select the image and in the Object menu lock the image (to avoid moving it by accident).

Select the pen tool and set up with no fill color and green stroke (or whatever color that you can see contrast with). Set up the stroke in 1pt or less.

Draw with the Pen tool

Draw straight line segments with the Pen tool

The simplest path you can draw with the Pen tool is a straight line, made by clicking the Pen tool to create two anchor points. By continuing to click, you create a path made of straight line segments connected by corner points.

Now go to the view menu and activate the snap to Grid

These options are essential to draw good lines with perfect intersections.

Now you have to start following the shape of the head around the edges, it is very important to do every click in to the grid intersections points.

Draw all the different parts of the face and expression lines

Create triangles; that means that every part of the face has to be a rectangle and no other polygon. Now create lines to connect every line and create a triangle

Live Paint Bucket

Paint every rectangle with the correct color. There is one easy way to do this in Illustrator, you can use the Live Paint Bucket on this example. Select all the lines and select the live paint bucket tool, then hold down the option key (alt) to switch to the eyedropper tool and select the color from the picture behind the triangle that you want to paint, release the option key and click with the paint bucket tool.

What are you pathfinder options set to

Just wondering if using live paint works to color the triangles instead of using pathfinder tools?

Hard to judge without taking a look at a sample .ai file.

Also, which version of Illustrator are you using?