Just because they are active don't mean they are gonna water your tree. If you want certain people to water your tree make sure you waters their every day and get enough regulars so your tree is full every day.

Also get higher friendship with people, higher lvl friends give 2-3 points per water.

Since there's a limit to watering friends' trees, some people tend to cherrypick who they'll water. Some just auto water from their status list so make sure to water the really active ones so your name gets on their list.

Work on getting a number of high level friends.
Personally, I like to water those with whom I have a high friendship level, cause top 3 I use for the commission thing and by focus on watering a few ppl of high relationship, it's more likely that it goes toward future top 3s.

Also, for the harvest exp, friendship level increases the amount of exp gained by water. I think it's every 3 levels or something. So if a lv 13 friend water you, you get like 5 exp, which is a quarter way done even if u have high lv tree. Just a few of those will get your tree grown.

Watering caps out at 3 points per person. Lvl 15 friend (max level) still gives 3 pts.