I've got the same problem. Had to start the next level as a new game

Make sure you install all Levitation maps and enable them all in HL-A I was having the same issue until l did this.

Same problem, but all maps downloaded and enabled in game. I even restarted Steam VR - didn't help

Same issue for me :(

Solved enabling all parts (in addons) and then making a new game

Yeah, I have all addon parts enabled, still broken

When I ran into this issue, I first tried to verify or reinstall the workshop items. Turns out the information which add-ons are activated is stored on your savegame, with no straight forward way to activate additional addons. So yeah, you have to enable all parts before you even think about starting your first game :D
But anyway, losing roughly half of the resins you found in the first part is not really a big deal, and with plenty of ammo for the shotty, I don't even care about weapon upgrades. But it could be explained in a more prominent spot that you have to enable all parts before starting a game.

i literally tough that part 1 is 4-5 hours long and then part 2 is also 4-5 hours long and so on lol

@Avocado, I spent around 4 hours in Part 1. I was exploring the areas where we fought combines, admiring how authentic it feels, as it is a proper DLC from Valve. I assume I would spent the same amount in Part 2.
This is really, really solid work.