The question does not give sufficient specifics. I'll just take a few swags at it...
I'm assuming that you have not observed any fuel leaks and do not detect the smell of fuel when starting or running the engine.
Also, in all the areas below, if you are removing a filter, first clamp the fuel lines to and from the filter to prevent siphoning and the loss of excess fuel.
1. I assume what you are calling the pre-filter is the fuel/water separator filter. If that filter has not been changed in the last year or so, change it. It is designed to filter water and any gunk from the fuel tank out of the fuel before it gets to your engine. They do get clogged up over time. In the future, drain the filter regularly - like after every 20 hours or so of running. There is typically a wheel valve on the bottom - you open that up and allow a couple ounces of fuel/water/gunk to drain into a container that will fit under the filter. If you did change that filter recently, did you put a couple drops of oil on the seals when you installed it? If not, you might want to remove it, check the seals, and re-install, lubricating the seals in the process.
2. Whether or not you change that filter, make sure it is screwed in tight and that the fuel lines in and out are tight.
3. Next in line after the F/W separator should be the pump up bulb. With the engine off, make sure you can pump it up until it gets hard, and it should stay hard until you start the engine. If it doesn't do that, then check valve is likely shot, so replace it. BTW - that pump-up bulb should be oriented vertically, or near vertically so that the fuel line going to the engine is above the fuel line coming from the tank.
4. Next, go to the point where the fuel line feeds into the engine. Make sure that coupler is seated tight and not leaking. Also, check where the engine's internal fuel line attaches to that coupler and make sure it is tight and not leaking.
5. (Here, I'm assuming that your 150 is a lot like my 225) The first filter in line on the engine will be the internal fuel/water separator. Drain the bowl and re-install making sure that it is seated properly IAW your Owners manual. Lightly lube the seal with a drop of oil. If there are two wires coming into that filter, then you have the type with a float switch in it. Make sure the float easily moves up and down. Next is the low pressure fuel filter. Remove it IAW your owners manual, and drain it or replace it if it has not been replaced for awhile. Also lightly lube the seal with a drop of oil.
6. If after all these checks, and the problem persists, find a piece of clear plastic hose with the same inside diameter as your current fuel line. Install that hose between the low pressure filter and the low pressure fuel pump. Start up your engine and observe whether you see any air bubbles running through the piece of plastic hose. After this line of diagnosis, you have pretty well eliminated an air leak as your problem.
If the above does not uncover the problem, then you MAY have a faulty low pressure fuel pump. The only way to tell for sure is to test the fuel pressure, which should be done by a professional. Those pumps are quite reliable. Over the last six years or so, few people on this forum have had any problems with them.
As a final thought, when is the last time you changed the high pressure fuel filter? I don't know about the 150's, but on the 200's and 225's, the high pressure fuel filter is the source of many fuel feed problems.
