What is the ruling in the case of a da'ee who calls people to apply a practice which he is not able to apply himself?

Allah admonishes and reprimands a person who calls to Allah, calls to good, and does not do it himself. Allah says:

أَتَأۡمُرُونَ ٱلنَّاسَ بِٱلۡبِرِّ وَتَنسَوۡنَ أَنفُسَكُمۡ وَأَنتُمۡ تَتۡلُونَ ٱلۡكِتَـٰبَ‌ۚ أَفَلَا تَعۡقِلُونَ

Enjoin you Al-Birr (piety and righteousness and each and every act of obedience to Allah) on the people and you forget (to practise it) yourselves, while you recite the Scripture (the Taurat (Torah))! Have you then no sense? [1]

If we find a da'ee calling to Allah while he does not practise what he preaches, he is indeed in a very dangerous state.

Prophet Mohammed (ﷺ) describes this person condition on the Day of Judgement as:

“A man will be brought on the Day of Resurrection and thrown into the Fire; his intestines will spill forth in the Fire and he will go around as a donkey goes around the millstone. The people of Hell will gather around him and will say: O So and so, what is the matter with you? Did you not enjoin what is good and forbid what is evil? He will say: Yes, but I used to enjoin good and not do it, and I used to forbid evil and do it.” [2]

However, there is a possibility of a person calling to a certain practice or to a certain act of worship but then he finds a better act of worship or something better that will please Allah. In that case, there is no blame on that person who leaves one act for something better. Allah will reward him for that.

References:

[1] Surah Al-Baqarah, 2:44

[2] Al-Bukhaari (3267) and Muslim (2989)