Content reference: demo v0.5.4
Read the actual coverage display
The bottom-right forest fill statistic was added in v0.5.0. Use the numerator and denominator shown by your own game where available; a screenshot’s map total is not a universal constant. Players have reported stopping at 99.9965%, so a percentage that looks almost complete can still represent a small uncovered patch.
Search systematically
Sweep the outer edge, then pond edges, the areas beside shops and gaps around large plants. Move slowly and use the appropriate unlocked plant or Eraser rather than repeatedly running the same central circuit. Refill water and stop sprinting before trying to plant. White blocked-radius rings can explain why an apparently open area rejects a tree; Happy Tree Friends changes the relevant placement restriction.
Check version and objective
The v0.5.0 patch fixed several unfillable gaps around objects and adjusted paint radius, so early screenshots may show different behavior. It also notes birds appearing when the forest is fully filled. Do not promise automatic gap filling from an old developer suggestion: a proposal is not proof it shipped. If the statistic still does not move, note the build, location and selected plant for a reproducible report.