low slope leaking
Well, I've seen a million of them like that. I would say that you made some kind of a mistake at the transition. The most common reason I have seen for this problem is when the lower slope roof is the only piece that is being done and the tie in is done wrong. Very common is, too much head lap on the last row of shingles before the change in pitch. That is easy to see if you just look careful at a horizontal shingle joint, how much headlap headlap the shingle below has,I have sometimes seen wood where there should be an inch and a half of headlap. I have even seen cases where the end laps of the 2 transition rows lined up and shingles will not work like that. I never used any metal flashing in a case like you have there but that could be the source of the problem also. If you were tying the flat stock into the existing upper slope you might not have got it far enough up under the existing shingles, that is just another lack of headlap problem. You need to take the nails out of TWO rows of shingles when you install a flat stock flashing to insure the flashing is up underneath far enough. Get up there with a window bar and a roofing hatchet and you will find the source of the leak right away. When I had roof jobs like that to do my men would always go up the steeper slope 5 rows of shingles. Used a few more shingles but never any leaks. You said you used ice and water shield. Did you cover the entire low slope with it? If you didn't and in a worst case senario water is backing up under all the shingles that are not underlaid with ice and water. This doesn't usually happen on that pitch but I have seen it happen many times depending on the volume of water running down the roof which is dependent on the size of the connecting area and the volume of water running down it.