I have problems sometimes with the sprint/snap key getting stuck in the held down mode. Wriggling a few keys together normally frees it up (eventually), otherwise a quit and restart.
However, this doesn't mean it's Valheim specifically. It could be the keyboard* itself, or a rare problem in the Unity Engine I once read about somewhere.
*For instance, in my case, did the problem start after I updated the Keyboard firmware, or when I changed the polling rate from default to 1000Hz - can't remember?

Originally posted by A Dismayed Cup:

Walls can still be snapped to their foundations, and beams to some walls, but some things can't be snapped. It's very confusing. Anyone else experienced this?

But this seems to be related to the quirkiness of the system.

Example: Had a nearby core wood structure holding distillery barrels preventing me from embedding a stone slab into the ground, but lowering the ground more than usually needed allowed me better access to snap grid of the floor to sink it in.
Using alternative building parts can also help with problem solving and coming at it from a different direction - i.e. sitting down to get underneath something, scaffolding....

it has gotten a bit dirtier. However a leading cause also is interference from other objects interrupting the plane. The engine currently is having a great deal of trouble with multiple objects on the same or close snap-points, or being 'blinded' by intermittent objects.
One easily repeatable situation is crossing beams, where a diagonal one stretches through where a horizontal beam is being placed. Large X-members display the same behavior, it is quite tell-tale. Or multiple 45's getting attached to a vertical beam at the same point. I can hassle with one snap-point for an entire in-game day to get it to finally land. Sometimes its easier to destroy other stuff and then rebuild it. It certainly could use some finer tuning, along most of the other tools.

Snapping is difficult, but as with every other task, practice makes perfect. Try always to move the mouse to a leading edge, then usually the part will snap. It needs practice.

Snapping can be weird sometimes when the surface you're pointing to and the ideal square shapes of the snap grid don't really match up. The game treats the build objects as simple cuboids and rectangles, but refers to their more complex appearance to decide where the mouse is pointing.

Originally posted by Leffe-(SWE):

If you had mods or haven't played for a long time the usual fix for the game not starting or behaving strange is to uninstall the game and manually remove the Valheim folder before reinstalling again.

Woah! That is a drastic solution and not likely the problem. He's a noob and it seems to me that the problem is more likely that he can't see the snap points from where he's standing. If you can't see the snap point you can't put the cursor on it. You often need need to build temporary scaffolding (stairs and floor pieces, etc) to get yourself where you need to be to see the snap point.

I'm having the same problem when building. There is no snap to.

If you had mods or haven't played for a long time the usual fix for the game not starting or behaving strange is to uninstall the game and manually remove the Valheim folder before reinstalling again.