Hoppers will not lock when they are part of a transparent block tower with redstone dust powered only on the side of the hopper. See Figure 1.

Steps to Reproduce:

1. Place 2 glass blocks or slabs as shown in Figure 1.
2. On top of the glass blocks or slabs, place redstone dust
3. On top of the lower redstone dust, place a hopper as shown in Figure 1. Do not place redstone dust on top of the hopper.
4. Power the redstone dust tower from the bottom only.

Observed Results:

• The hopper does not lock (as shown through a resource pack or through setting up a hopper clock).

Expected Results:

• As the top redstone dust is powered and facing into the hopper, the hopper should lock.

Notes:

• If the top redstone dust is powered from a power source which is not part of the transparent block tower, the hopper locks even if the power reaching it from the bottom of the tower is stronger (See Figure 2). 
• The bug appears to be non-directional.