When taking a swing at lump for the first time (actually, for the first few times), you may need to spend some time getting your WSM / setup / routine "dialed-in" AGAIN.
It is likely that lump will behave a bit differently from briquettes.
Some further suggestions:
Once you find a brand of lump that you like / can readily obtain, stick with it, and do not switch brands until you learn how it tends to behave.
When burning lump, get the second charcoal grate and place at 90-degree angle (rungs) to the original charcoal grate. It keeps the small, lit stuff from falling-through and helps to keep the fire going.
Store it off the ground, in a place where it will not get damp. Mine is in the detached garage, on a shelf or in the garage "attic".
FWIW - I had bad luck with Cowboy, both in a regular grill and in my WSM. Been using R.O. and Grove, which seem pretty similar.
When using the water pan, I usually get somewhere around 8-hours out of a ring packed full of lump - although it sometimes starts to lose some heat around 4 to 6. sometimes, I just need to give the coals a stir, which will knock dust out of the bottom, improving airflow, and bring hot coals into contact with un-burned.
My usual ritual:
-Standard Start, full or 3/4 chimney of lit on top of packed ring with smoke wood burried / some on top (I have not tried using lump to start, but I've been thinking about it)
-Vents nearly open for the first 30-minutes to hour - let it settle-in.
-I usually try to orient the lid vent down-wind, to draw heat out of the top and pull air through the smoker.
-Then, depending on temp / wind, I adjust the vents to 30 to 25% open, give it another 30-60 minutes.
-If she's holding steady, I can leave it alone for a while.
-I have a nice, big, tell-tru thermometer with 4-inch probe that I stick in one of the top vents. A quick glance at it (I can see the needle from upstairs windows if I need to) lets me know if I'm "In the zone". I've debated mounting it permanently, but I like that I can "aim" it, simply tilted through the top vent hole, depending on which way the top vent is pointing.
I don't fret if my temperature is within about 30°F of my target.
Hope this helps