Manual Transmission getting stuck in gear or won't go into gear
 

I bought my 13 BRZ last year. It sat for awhile while I was fixing somethings, but these problems have existed intermittently since I bought it.

Sometimes the transmission gets stuck in gear after it's been parked. Sometimes it's virtually impossible to get into neutral, other times it's just very stiff. Typically after that, it's pretty much impossible to get into gear. It just won't go. I can let the clutch out in neutral rev it up a little, clutch in, and it's like I'm hitting a wall. Usually after sitting/fidgeting for 15-20 seconds, it'll suddenly go into gear (usually 3rd or 4th) and then it will go in the rest of the gears fine.

I have an IRP shifter, and for awhile I thought it was related to that. After watching several install vids, and looking at how it's installed in my car, I'm having a hard time understanding how it could be related to the shifter itself.

I tried searching, but couldn't find much reference to this other than just the clunkiness that can exist shifting between 1st and 2nd, and it's definitely not that.

Anyone have any ideas? I'm planning on replacing the trans fluid this weekend to make sure it's not related to that somehow, or to see if there's any crazy shavings or anything in the fluid. Beyond that, I'm kind of at a loss.

Check the adjustment of the clutch pedal.
I'll blame that IRP shifter all day though- put a stock one back in and see how it is.

If it's the shifter, and I'm not saying it definitely is, but if it's the shifter it's probably because the stock shifter bolts to the transmission and moves with the transmission, whereas the IRP shifter bolts to the chassis and does not move with the transmission.

If it shifts fine (barring a bit of dog misalignment in some gears) with the engine off, the clutch is not fully disengaging. If it has the same issues with the engine off, it might be the shifter. As noted by @pope, the IRP bolts to the chassis instead of the transmission via the carrier, so a misadjusted IRP shifter can result in impossible shifting. Follow the instructions online on how to adjust it.

I've had the disk lock up to the flywheel when one of the damping springs exploded out of its pocket and got sandwiched between the two.