How does the situation described in the first paragraph occur? I don't think I could have given it a good explanation or any at all until I looked very closely at the words of Jesus in Mark 4:23-25. Let's let Him explain it for us:
23 If any man have ears to hear, let him hear. 24 And he said unto them, Take heed what ye hear: with what measure ye mete, it shall be measured to you: and unto you that hear shall more be given. 25 For he that hath, to him shall be given: and he that hath not, from him shall be taken even that which he hath.
What Jesus had told the multitudes (v. 9) He now repeated specifically for His disciples—they could hear, so they needed to use that ability by paying attention to what Jesus had just said (v. 23). He expands on the "ears to hear" theme with them. The degree to which they gave heed to what He said would be the degree to which they would profit from it (v. 24). God would graciously bless attentive disciples with even greater benefit than the effort they expended in heeding His words; their blessing would be disproportionately large. The spiritual results which the disciples realized were to be based upon the amount of effort they put forth; they would reap as they had sown. The one who has learned spiritual truth and applied it diligently will receive even more truth to faithfully apply.
If a person works hard to obtain something good, he or she normally receives other good things in addition (v. 25). If a disciple pays attention to and assimilates the revelation God has given, God will increase his or her capacity to understand and appropriate more revelation. However this principle works the other way too. The person who does not use his or her ability to understand and respond to God's revelation appropriately loses that ability.
These verses say that even His disciples will not receive some of His truth. They're either taught wrong and are not using discernment, they refuse to listen when told, or they won't start practicing or proclaiming what they now do know. Many of these sadly are pastors who have a loyalty to a non-Scriptural institution or a group of friends above God and His Word. Instead of submitting to Scripture, they stick with private interpretation, sometimes buoyed by an author that will footnote their position. They hold on to the teachings of men instead of God. Pragmatism is often the influence. Whatever may be the cause, as a result, they remain deficient in Biblical understanding. They range from evangelicals to fundamentalists and everywhere in between. They say they are believers, but they don't get it.