What device are you playing on? I have accidentally figured out that playing in a narrow browser window hides the right side of the board without any hint it is there (except for the health bar being cut mid-heart - I have initially thought remembering whether you already have 15 hearts or not was part of the game process). This partially visible/explored board might be the reason you did not get enough resources for lvlup
I played later, still I thought perfectly, but was 4-5 hearts short. Then I played again and cleared the board with 2 hearts to spare, and another time with 1 to spare. I've scored 303 and 302 points. It might be possible to score higher by getting really lucky.
I think it's about delaying using the heart-healup as long as possible. Use them at later levels, not earlier levels, whenever possible.