Fear can be one reason. Lear to overcome it if you have any fear, which is something that blocks you from seeing spiritual beings. When your brain has any negative emotion about something, it basically blocks it to be seen.
Your chakras might be clogged/unbalanced. Learn to balance them or unclog them. Find tutorials online.
If you saw spiritual being in your youth and now you can't, it means that you have to WORK and accomplish different level in your spirituality by yourself.
Meditation is helpful as well, but basically that's not all. You need to balance yourself and stop fearing if you do.
Firstly, one reason, as sue said, might be blocking yourself due to a fear, which is something that might make the process of sensing them harder. While being scared, you actually kinda block yourself from seeing them and make yourself to avoid the experience of sensing them.
So the first reason of having your difficulty with seeing spirits might stem from your deep fears of dealing with spirits and experiencing anything related with them. So shortly, you can't see spirits because you are afraid from them. You probably heard so many frightening legends about ghosts so that you prefer not to see them and believe that they do not exist. Consequently, your mind blocks you from seeing them. So basically when your mind blocks any sort of sight or entity, you will not be able to see it or sense it.
In order to get rid of this difficulty, you need to remove your fears and overcome them. One way to get over the fear is to be aware of your mind's fear from ghosts until the fear is no more. Do not fight the fear, accept and observe it. My tip is that.
You can also check why you are fearing of ghosts and watch your mind's conflicting forces of underlying skepticism and underlying belief. These are the causes for your fear - on the one hand you believe in ghosts and therefore want to avoid them, on the other hand you are skeptic and therefor feel that you are isolated and alone in the world and therefore can trust nothing to help you and guard you, hence fear.
Observe in particular your skepticism, observe and accept, do not fight it. It will also become weaker as you observe it.
When the fear and skepticism become weaker, the mind's blocking of non-skeptical phenomena will subside and the reality created and observed will start to include more happenings that cannot be categorized as natural, logical, science-proven. In this new state of mind you may be able to sense the presence of what you call 'ghosts' (spirits).
I also saw that someone told you that you might have a third eye chakra blockage, which is not necessarily true. The fact that you cannot use your third eye chakra in order to see spirits does not necessarily tell us that you have a blockage in your third eye chakra.
Mostly when this sixth chakra experiences a blockage, one may suffer from frequent nightmares and headaches, poor eyesight, lack of concentration and sometimes even migraines.
Also if the 6th chakra is not balanced, you have the potential to be a cold intellectual who trusts only reason and logic and is manipulative when it comes to getting what you want. Also when you say that , you are undisciplined and probably an underachiever.
So if you don't find any of this symptoms this may just tell us that you haven't developed it enough and didn't practice and meditate in order to improve your third eye chakra and develop your skills with it.
So what you simply have to do, just as about any other skill or ability in the craft or other subjects, you have to have patience for it and practice in order to develop it and improve it.
What I recommend you is to use meditation in order to improve your third eye chakra. You need to dedicate time, effort, energy and patience in order to achieve improvement and development of this chakra.
Through performing a third eye meditation you can open your subtle senses to perceive higher-dimensional worlds and energies. The third eye is the center of what is called clairvoyant sight , meaning "clear vision," and refers to the higher-sense ability to intuitively see or perceive subtle energies.
Just dedicate some time to meditate and give it as much time and effort as you need in order to success.
You may search about opening your third eye in Google (or any other searching engine) and look for the sixth chakra (third eye) meditation methods. You will find a bunch of articles about it, and you will probably get the information you need.
There are different methods of sensing and seeing spirits and other entities. As diablo said, you need to work on it and choose either of the methods.