It's been live for a week, has good reviews, but not a lot of downloads. Right? Ads degrade the perceved quality of the game and rely on a large number of eyeballs to generate any meaningfull revenue. Haiving a free app will increase your installs greatly, but in my experience ads generate pennies on the dollar relative to in-app purchases.

I'd consider adding cosmetic and functional items to the game and an in-game currency (gold) which they pick up while playing or can buy in packs for real money.

At the start of each level i'd show the player which items may drop within the level, including about five or so options and have them range from epic to basic with the awesomeness linked to their drop rate. When the level is completed give the player one of the possible rewards and give them the option to re-roll for gold, with each re-roll costing more than the last.

You could put the item in a chest and include it in the puzzle in a way that makes collecting it the harder option so when they open it at the end they feel that they've earned it and will put more value on it (which increases the value of a reroll)

Excess in-game currency could be spend directly on chests, with each chest containing a single item and larger chests containing the better items (and costing more gold)

You could also give away chests as part of the act of purchasing gold for real money "$0.99 = 100 gold & 2 small chests". This entire system hindges on the items been something the player wants and evolve the gameplay for them. Existing customers would get gold equal to the cost of the game + one item which is ONLY available to those "founding" players.

You could double down on this by adding lives, have them regenerate slowly over time, buy more with gold, have some items that can be found that increase the regen rate and life capacity.