Over time the L5450's fuel pickup pipe in the tank's bottom cup can get clogged with debris. The tank also has 2 vent lines which will allow air in if the clogging develops too much suction in the pickup pipe.
When my L5450 gets air in the fuel line, I take off the feed line at the filter and give it a brief shot of compressed air, back-flushing the pickup pipe. Access to the pickup pipe at the tank end is essentially impossible without cutting the tank apart - giving it a puff of air at the filter end is much easier.
Years ago when this problem first showed, I noticed there are 2 vent lines on the side of the tank - not sure why. I tried just connecting them to each other, to rely instead on fuel cap's vent. It worked, but the root cause was that the pickup pipe tubing has a sharp edge on the end that had snagged some small fibers, eventually restricting flow. The pipe is under a welded baffle in the cup, so no access.
After puffing it back with air, it should remain free-flowing for months, maybe years. Hope this is useful, Dick B