Is this your first game (or game of this type if you have worked on PBBGs or something else before), or one of your first? If so, that is entirely normal, and I would suggest to simply move on to bigger things. Very few developers make games that really hold people's attention on their first outing
If it isn't, then it likely just needs some things to spice it up. "Fun factor" is elusive and hard to pin down. Heck, even many commercial studios can't turn out fun games and they have sometimes millions of dollars at their disposal! Try to think of some unique angles to add to it that you haven't seen before, and then tinker with them until they feel right. Sometimes it is just the feeling of the game that is off and needs some changes.
However, some quick suggestions on common ways that games can be fun is that they move fluidly between different different tasks of play, they can have multiple ways to meet an objective or multiple objectives to meet, they can require balancing different tasks such as collecting power-ups to keep up attack power needed to keep steadily encroaching enemies at bay (preferably moving smoothly and easily between tasks, as mentioned above), they can have variety in the order in which things happen, they can have online leaderboards to encourage competition, etc. Just think about what makes some games you like fun!
Its my style 
