Sometimes the light in my spiritual eye is pulsating or strobing. What is the reason for this?
Any fluctuation of the size, color, movements, and so on, in the way you perceive the spiritual eye generally mean that your mind at least somewhat restless and won’t settle down to be perfectly still. What you see (or don’t see) at the spiritual eye is generally a reflection of your mental state.
On the other hand, it is wonderful indeed that you are seeing a light at the spiritual eye at all, no matter what it is doing! Many meditators don’t see anything, or very little and even that rarely. So do be grateful.
Pray for help and guidance and keep practice your meditation techniques. It will probably settle down and appear to you in a clear and perfect state, after a while.
And never judge your own or anybody else’s spiritual progress by the phenomena experienced in meditation. Yogananda says this is not the true test of effective meditation. What IS the true test is how you are changing for the better, as a person, in daily life. Observe that as the first and most important thing.