Complacency is a sin. It implies that we do not trust God enough that He can make things better. It is doubt in His power, in His complete control over circumstances, in His infinite ability to affect all things.
It also implies that we do not trust that He can change us, that He can make us better, that He can empower us or enrich us. When we are complacent we are showing that we do not appreciate what He has given us, what He has made us into, that we do not acknowledge His blessings, both the gifts that we already have, and those that are in His capacity to give.
Being complacent is different from being content. A sense of contentment comes from a job well done; it comes at the end of an ordeal or difficult undertaking, or from achieving something, or finishing something through to completion. Complacency is not finishing something at all, or stopping in the middle of a challenge, just losing steam and stopping.
The most common cause of complacency is a loss of or lessening of spirit. For some reason or other, the passion dissipates, and although that is not an excuse, it is usually the case that passion is mistaken for spirit, and once that is lost, everything is affected.
Another cause might be discouragement due to some outside force or other. Be it a challenge or some temptation, a weak spirit might be easily poisoned into stopping the good work.
The cure for complacency is diligence in just pushing through, and just enduring. Pray for God to renew your spirit. Pray for healing for that which is broken. Pray for God to open your heart and your mind once again.
And do not be complacent in your belief that He will answer when you call.