Hey again @Vicktorious09! As ShadowWolf said, the SLVL in a room is determined by the 20 rarest items that are placed in it. You can still place more than 20 items in a room, but the SLVL mechanic will only factor in 20 that have the highest rarity. The other thing is that the max SLVL for each room is 70. The way that you increase it to 100 is by receiving 3 profile interactions from other players (the "Play Lottery" button). Each profile interaction you receive gives you 10 SLVL (up to 30), which lasts for 24 hours.

As for getting your rooms to reach 70 SLVL, I'm garbage at math, but you can check how much SLVL a deco item will give you by tapping or hovering over the info icon in your inventory, like this:



I notice that you have some 0% rarity items placed, which is perfect because they give 5 SLVL (which is the highest an item can give). It takes a while to get enough rare items that add up to 70, so the only thing I can recommend is to keep hoarding! Potions can also help a lot; in this case if you pair a Decorative Item Potion with an Item Rarity Excluder, you're much more likely to find rare decorations.

I feel like I'm forgetting something, but I've already rambled too much lol. Please let me know if anything I said was confusing or if you have further questions!