They don’t want to be misunderstood
According to psychology, they do not want to let you know that they actually like your picture. Even if they do, they refrain from liking it as they do not want to let you know.
They are not close friends
Some people always like their pictures of their friends, relatives, and acquainted people. They do not hit the like button for the person they do not know much about.
They don’t like the picture
Some people actually do not like your picture, so do not press the like tab. People who regularly like your posts may not like your one post, and that’s why they did not like it.
They un-followed you
Some people might have unfollowed you, due to which they are unable to see your post and, subsequently, like it. (See Why you are suddenly unfriended on Facebook?)
They don’t like something about the picture
You may also have posted your picture with someone who is not liked by most people on your friends’ list. That may result in decreasing your likes.
They did not see it
Some people might have missed your posts, and if they had seen it, they would have definitely pressed the like button.
You don’t like their posts
Some people believe in equivalent retaliation. They will like your post only if you like theirs; otherwise, they will not intentionally like it. (See Why some people never like your posts on Facebook?)
They don’t come online
There might be many people on your friend list who do not come online often. They will not get a chance to see your post and hence, like them.
They are jealous of you
One reason people do not intentionally like your posts even after seeing them and admiring them is that they are jealous of you. It may happen if you are a renowned personality or a public figure or too good-looking.
They think you have enough likes
Some people think that you have enough likes on your post, so let’s not go for it. They think that it will not make any significant difference even if they like your post.
They are not in the mood
It happens with many people that they like many posts only when they are in a good mood. Otherwise, if they are not in the mood, they won’t like hardly any post.
People who keep changing their profile pictures can be insecure, lack confidence, and can't make decisions easily.
Can I tell who's looking at my Facebook profile? Is there a real way to see who views your profile on Facebook?