Full inspection reveals many problems.

These shingles are beginning to slide down the roof, marked in pink. I looked into it a bit better, discovering the reason why.

The reason was that the shingles have been high nailed, meaning when the shingles were nailed down, the person nailing put the nail in way too high and missed the nail line. I checked this area along with many others and found high nailed shingles all over the roof. A shingle has a nail line, and that nail line of the shingles is thick with 2 layers. Above that line, the shingle is thinner with one layer. Most nails will shoot through or almost shoot through the shingle. Here are some pictures I took

there are many other pictures I took. Too many to list here. 

These cracked ridge cap shingles developed a hole in it. This was likely the cause of the leak in the middle dormer.

Exposed nails can cause the nails to rust away, leaving a hole for water to penetrate the roof. These need to be re-caulked

Moss is terrible for your roof. Moss has a root system that is thin as hair. It grows in the dark (in-between shingles), letting water wick under the shingles causing nails to rust away.

The shingle union is way too close to the corner of the dormer. This can cause leaking at the corner.