I've been installing 5" hickory solid wood flooring in various portions of my house.

On the second floor I installed flooring in the hallway which leads into the master bedroom at the end of the hall. Once in the room I made the first row near the center of the room and went all the way in one direction. Then I went and did the reverse direction. Here is where I think I made the mistake...

I thought I had read that when you put in the slip tongue you don't nail that side because it splits. From some reading tonight it seems I might have been wrong...

So when I installed in the other direction I glued the slip tongue in place and then installed all the way to the other way with normal blind nailing. That middle seam has no nails.

I have been putting a serpentine strip of adhesive on the back of all the boards, because they are 5". I assume I now need to go back and add nails that seam.

Do I just face nail every 8" or so on one of the strips?

Is it reasonable for me to try to angle nail down into the joint between them? It is a scrapped finish so the nail holes might not be that noticeable.

Are there any other solutions I should consider?

Slip tongue should be no more prone to splitting than a regular tongue. I'd leave the seam alone for now and see if the adhesive keeps it secure. You can always go back later and do your face nailing. Consider countersunk trim head screws instead of face nails.

Thank you for the feedback.

Earlier today I went down the seam between the boards and used my smaller nail gun (18 g brads) and put them every 6-8". You can't tell that they are there at all.

I'll hold off on anything else for now, but what should I look for? That middle part peaking up?

Squeaks and movement. If you do need more nails or screws use something bigger than 18 gauge. 15 gauge or hand driven finish nails (predrilled). Driving the nails at a slight angle will help.....2 nails on each side of the board driven at an angle toward each other. Countersunk trim head screws are good but will require filling a larger hole than nails.

Understood. I've been using larger everywhere else. I just used it as a starting position as I knew it would be inconspicuous / easy to hide.

Movement would show up as gaps between boards or buckling?

Seasonal changes in the house humidity levels would cause boards to expand (higher humidity....Summer) and shrink (lower humidity....Winter) widthwise which would make the joints between the boards larger or smaller. This is normal. Buckling or cupping boards would be indicative of a high moisture problem or an imbalance in the moisture level above and below the boards. Really high moisture in the house or water damage would cause the boards to swell and push against each other and buckle. A moisture imbalance such as higher humidity in a basement or crawl space vs the humidity level in the room above would cause cupping.
If you feel movement (up and down) or hear squeaks when you walk in this area that would mean the boards are not tight to the subfloor and would need the nails or screws to tighten them down.

A great response. Thank you.