First, look at the spring from sliding dispenser cover - it has 2 ends with loops - one end has a little "curve" in it; other end is "straight", no curve.
Sliding cover and lever of dispenser have little knobs for attaching the spring.
The loop of "curved" end has to be pushed and attached to sliding cover,
the loop of "straight" end has to be pushed and attached to lever of dispenser (the lever is blue color, other parts are gray).

After spring is attached to both - sliding cover and lever, push the cover first at upper groove, then at lower groove - It should slide easily.