Is it actually possible that an average person in these troubled times can become fully Krishna conscious? If so, why do some people lose interest in their practice, even after practicing for some time?
First, it should be understood that anyone who takes up the process of Krishna Consciousness is not an average person, but an exceptional person. Krishna explains in the Bhagavad-gita that only one out of many, many persons looks for spiritual solutions and tries to connect with Krishna. Lord Krishna says that such a great soul is very rare.
That being said, this material word is quite problematic and it's not possible to overcome it by ourselves. We need the help of guru, Krishna and other devotees. By seeking guidance from guru, and taking shelter of the holy name we can make advancement and become aloof from the sufferings of the material world.
Also, material things need to be kept in perspective. For example, this material situation is temporary. Whatever happiness or suffering we're experiencing now will go away in short order. If we look back in our lives, we'll find that what once seemed like a very serious problem seems insignificant a few months or a few years later. Life goes on. Happiness and suffering continue to come and go. We should try not to get too involved in these things and realize that our ultimate goal and happiness will come from connecting with Krishna.
When the senses—particularly the mind—become attracted to material things, we tend to falsely think that we can find happiness from eating, smelling, touching, playing with, loving, or otherwise interacting with material objects. We can then lose interest in our chanting and spiritual activities and drift back into material illusion, only to wake up and find ourselves once again sad and wondering, "what am I doing here?" Then, if we're lucky, we again take up the process of spiritual life. Hopefully after only a short detour.
Keeping good association, reading and chanting regularly, taking guidance from the guru, and living a life guided by the principles of bhakti-yoga will help us develop strength and determination.
I hope this is helpful.