poly finish problem

[Shogun]

I am working on a knife display case for a guy and I'm having problems with the polyurethane. The wood id oak and sanded very well and stained with minwax oil base stain. I am using the recommended same brand poly but it seems to "float away" from the grain leaving it dull compared to the non grain area of the wood. The second coat of poly makes it look obvious. I considered using sanding sealer in between the stain and poly but the instructions say it's not necessary. I am trying to achieve that smooth "glassy" furniture type look. Can anyone offer any advice, or knowledge as to why this is happening or what I may be doing wrong? Greatly appreciated, and thank you in advance.

[Rapid Roger]

Oak is very porous and will soak up a lot of finish. I generally do a coat of shellac and bit of sanding first before staining or finishing any wood.
However, in this case, it is a bit late so, I would recommend to just keep adding coats of poly and sanding a bit with high grit paper or brown paper bag between coats. Eventually the pores will fill up with poly and you will get the shiny, smooth finish of your desires.
By the way, as you probably already know, the end grain is even worse as far as soaking up stain and finish. End grain is like a handful of soda straws and just keeps pulling it in. I usually sand end grain very well with a finer grit paper than the rest of the board and put on two or three coats of shellac before any stain or finish to "clog" up the soda straws ahead of time. It still may end up a different color but, it will be a lot closer to matching.

Rog

[Dakota Saw Dust]

If you are spraying your polyurethane, that has that floating effect particular with a heavy spray application. I use rub-on-polyurethane. It is hard [if not impossible] to apply to fret work, but but does a super job on case goods. Apply it with fresh cheese each coat and use 220 to 400 grit to sand between coats. I find using gloss has the advantage of showing you if your closing in on the smooth gloss finish. When you start off, after sanding, the hard closed grain while be dull and the open grain will still have a shimmer of gloss in it. As you apply 4-7 coats you will see less and less "shimmering gloss" as the pores fill.
I'll use rub-on-poly for the bases of some fret pcs.  I also like semi-gloss as a finish but will use gloss to start off with then switch to semi-gloss for the last coat or two.