Take your plug apart. EITHER something isn't assembled correctly, or you need to replace that rubber seal. It's been a few months since I've touched mine, but I guarantee yours have some issue, because mine are darn near "2 fingers" easy. I'm always baffled when I see guys on here with those type of plugs that are having issues, because mine are seriously dead nuts simple to get them in. The ONLY issue I've ever had is when the metal ear shifts out of position (from unlock to lock) before I actually have a chance to turn them. Julian's videos that others have posted in this thread are AWESOME for showing just how easy these plugs should be.
They're super simple to disassemble, you might see what's up with them right off the bat. For whatever reason, I can't find where I posted about disassembling my plugs. Mebbe I never did, so here's the pics of what you'll need to do to disassemble those plugs and inspect those rings. At the very least, you'll have to remove the ring to replace it with a good one, MAYBE you'll see what the issue is with yours. It's entirely possible that they botched the assembly of your cleanout plugs. If they could get them in, there's no telling what someone at the dealership could have done (some jabroni who was having problems and once his pushing and shoving got it in place, it damaged the rubber as it sat pinched or whatever)...who knows.
Mebbe add your location to your profile. No idea where you're at, but if you were near me in Wisconsin, I'd come over and help you out.
First pic (well, all of them, to be precise) is my plug. See how the skirt tapers towards the plug? Yours doesn't, mine does, mine is easy to twist in, yours isn't. Conclusion: Yours is damaged, cockeyed, or backwards/upside down somehow (although highly improbable. It needs a-fixin'.
This is what a plug looks like disassembled, and this is the only tool you will need for this project. If you need more than a screwdriver...
Step one is to remove your plugs from the boat. Undo these 4 screws (one already removed to demonstrate) and remove the stretchy tether to allow removal. You don't have to completely remove them just yet, do that later at an area where you can lay the parts out so you don't EASILY lose a small screw:
After all the screws are removed at your safeR location, remove the handle. You'll be looking at this (next step is to remove that screw in the middle):
Pull the washer off, set it aside with the rest of your parts - notice that bolt isn't all that long. Remove that metal plate carefully. You should have two pieces underneath that plate that can come out and easily disappear into the ether, never to be seen again:
Here's what's underneath that metal piece. Those two white "nubs" have springs under/inside them, and those are the pieces that may fall out. MAYBE something is missing from your plug that you've noticed by this point, maybe something is cockeyed due to all of this, maybe the bottom of that black "plate" could have been molded for crap (and ANY rubber ring will NEVER sit correctly), only way to tell is to disassemble it. Remove those six screws:
The white nubs simply pulls out. Spring may or may not come with it. Mine didn't, as shown in the pic after this one. The rubber ring maintained its position for mine, yours may simply fall off. I'd say that would be one indicator it needs replacement. Pull the spring along with the white nubs too:
By this point, if your rubber ring hasn't fallen off your plug, pull it off. Here's what you should have. I haven't looked at these since last fall, but off the top of my head, it'll be noticeable if the ring is on upside down. I don't believe that to be your problem, but compare yours to how mine looks. If yours looks beat up or wonky in some way, look at the rest of the plug, how it's molded, etc to see if something OTHER than poor assembly could have caused this. If the rubber ring slid off the plug when they were screwing it down, they could have pinched it, making it fold or bow out somehow, and the first person who was able to screw that cleanout plug into place sealed the fate of that plug until parts get replaced and that plug is remade back to the way it should properly work with a minimum of effort.
Assembly with suitable undamaged parts is the reverse. Really, Julian's video shows just how easily these should be going in for you. The Yamaha rep seems to be able to do this from an odd angle, even. I'll even admit, and you can probably tell from the pics, I've N-E-V-E-R lubed that rubber ring with ANYTHING, and it still works perfectly.
I want to thank you all for forcing me to do this (of my own free will), and forever damaging my cleanout plug by not being able to leave well enough alone! ?