I have been very moved by Pastor Armstrong's teaching, but I rarely experience the same impact when listening to my pastor preaching on Sundays. Why is there such a difference between his teaching and my pastor's preaching?
There is no clear biblical distinction between preaching and teaching, but we can offer some observations based on our experience.
First, teaching can be defined as expositing (or explaining) the meaning of scripture, while preaching combines Bible teaching with a specific call to obey the word of God. Proper preaching is dependent on sound teaching, for how can we obey what do not understand? Therefore, all sound preaching must include Bible teaching, though not all teaching must turn to preaching. The choice of whether to "teach" or "preach" will depend on the speaker's purpose and the setting. For example, Pastor Armstrong usually performs teaching when addressing a Bible study class, since the classroom environment lends itself to didactic instruction and dialog, while an exhortation to react in a specific way isn't necessary or appropriate to the moment.
Nevertheless even in such a teaching situation, the Holy Spirit remains constantly active in the hearts of believers to take the word of God as it is taught and use it to convict the hearer of sin and compel obedience to God's word.
Heb. 4:12 For the word of God is living and active and sharper than any two-edged sword, and piercing as far as the division of soul and spirit, of both joints and marrow, and able to judge the thoughts and intentions of the heart.
2Tim. 3:16 All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness;
In a different setting, Pastor Armstrong will consciously seek to exhort his listeners to think carefully about the teaching of scripture and consider a particular response. In this case, he preaches the message to encourage a specific response, which he feels properly reflects the teaching of the text. These moments are traditionally reserved for gatherings of the body intended for worship and exhortation (i.e., Sunday services, devotionals, conference sessions, etc.).
Once again, the proper preaching of the word depends first on proper teaching. If a speaker fails to teach a passage correctly (or to even address scripture at all), then their preaching will carry no authority nor propensity to move hearts. We suspect this is may be the issue you have encountered in your years listening to ineffective "preaching" in your church. If your pastor's preaching is not rooted in proper examination and teaching of God's word, then it is merely speech giving.