A bright band that dies inside the first two feet
This is the cosine term at work. Put a strip one inch off a wall and the ray reaching a point two feet below arrives at about 2.4 degrees to the wall plane, a cosine of roughly 0.04. At three inches it arrives at about 7.1 degrees, the cosine climbs to around 0.12, and that point gets three times the light. Check it with a meter on your own wall; the technique sits in grazing and washing explained.
A lit upper wall over a dark base
Sometimes the gradient is fine and the light stops short: the top four feet read evenly, then the wall goes dark. That is throw, not light fall-off, and setback and beam angle decide it together. A narrow distribution mounted tight covers a short strip of wall brightly; a wider one starts lower and never gets as bright. How that figure is stated is covered in how beam angle is read.
One brighter patch in the middle of an even run
A single hot area in an otherwise consistent run is almost always mechanical. The usual cause is a short length of tape lifted off the floor of the channel, sitting closer to the lens than its neighbors; a butt join where two lengths overlap does the same. There is a two-second test: press gently along the lens with the run on, and if the patch shifts, the tape is not seated. Fitters lose afternoons swapping drivers over a patch a clip would fix.
Light fall-off from one end of a run to the other
Everything above happens down the height of the wall. When the dimming runs along the length instead, brightest at the feed and weakest at the far end, the cause is electrical. Voltage falls as current travels along the copper, and on color-mixing products it can show as a color shift. The levers are feed position, conductor size and the maximum run published for that product; the rest sits in the way voltage drop shows itself over distance. One voltage runs the whole chain, so a 24V strip sits behind a 24V supply and a 24V controller, and a 12V strip behind 12V versions of both. If the remedy involves a wall dimmer, a hardwired driver or a junction box, that part is line voltage and belongs to a licensed electrician working from the printed instructions, from local code and from NEC requirements where they govern, with the circuit killed at the panel and confirmed dead first. Familiarity with low voltage is not a license.
A beaded texture inside the wash
From ten feet a wash looks smooth; at four feet a row of beads appears in it. That is the individual emitters resolving through the lens. A working guide, to be tested rather than trusted: the emitter to diffuser gap usually needs to be at least as large as the emitter to emitter pitch. LEDs and drivers generate heat in normal use, and a channel helps dissipate heat while holding the tape flat. No installation is obliged to have one. It reduces but does not guarantee that visible dots go away, and the rest of that answer sits with LED density, channel or profile depth, and diffuser type. At close range, check the emitter arrangement published for a product such as the COB 500 Series at high output.
A dark seam where two walls meet
An inside corner that stays black while both walls light evenly is not a fault. Light travelling parallel to one wall never turns to reach the return, so the corner sits outside every beam in the room. The fixes are geometric: carry the run around the corner, or stop short deliberately so the seam reads as an intentional break. For a second read, send a straight-on photograph of the lit wall, taken square on.
