When I've used lump in my WSM I find really packing it in there and shaking the bottom to make it all settle closely together helps and have had pretty even burns with lump. It does burn faster and for longer cooks I have had to add more lump. Maybe a little more information on exactly what you are doing with the lump could help to diagnose the problem. In my egg lump is 100% the way to go as the ceramic really makes it last a long time and burn even more even because it is so much more insulated.
All that being said I use Kingsford briquettes 99% of the time in my WSM and like the results. Some use lump, some don't. Nothing wrong with using either so use what is easiest for you
Shirley Fabrication Offset Awesome Pit
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Weber 18.5 Kettle
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