Thanks to you guys, I finally got both fireplaces to light for Christmas!
The problem with one was I had not known the procedure (see below).
The problem with the other is that the gas is shooting out too fast & the
sparks are flying in the wrong places so it won't easily light on its own.
Here is the procedure that works on the good fireplace:
1. Open the keyed gas valve about one full turn CCW (master switch).
2. Turn the large gas dial to the central position (pilot position).
3. Push the large gas dial in for about 10 seconds (pilot flow).
4. Push the small piezo-electric lighter a few times (light pilot).
5. Slowly release pressure on the large dial (pilot should stay lit).
6. Turn large dial 1 quarter turn CCW (the gas flow will audibly hiss).
7. Whoosh. The entire fireplace should light up in flame.