Teshuvah transcends human effort

ובמדרש ע"פ סכותה בענן לך מעבור תפלה.

And in the Midrash, on the verse "You covered Yourself with a cloud, that no prayer should pass through" (Eichah 3:44).

The Sefas Emes cites the Midrash on this posuk from Eichah, which speaks of times when a "cloud" blocks tefillah from rising.

שערי תפלה פעמים נעולין שערי תשובה לעולם פתוחים.

The gates of tefillah (prayer) are sometimes locked; the gates of teshuvah (repentance) are always open.

Chazal contrast two kinds of gates: the gates of prayer may at times be shut, but the gates of teshuvah never close.

כי תפלה הוא עפ"י עבודת האדם ועמידתו.

For tefillah is according to the avodah of the person and his standing.

Prayer operates within the framework of a person's own effort and spiritual standing - and what depends on human avodah can sometimes be blocked.

אבל תשובה הוא לזכות לנועם עליון למעלה מעבודת האדם והוא בחי' סוכה:

But teshuvah is to merit the supernal pleasantness (noam elyon) that is above the avodah of the person - and this is the dimension of the sukkah.

Teshuvah reaches a higher place that transcends a person's own efforts, drawing down a supernal sweetness from above. This is precisely the quality of the sukkah - Hashem's encompassing protection that comes as a gift from beyond one's own avodah, which is why its gates are never locked.

Summary: Tefillah depends on a person's own avodah, so its gates can sometimes be locked; but teshuvah draws from the noam elyon above human effort, and its gates are always open. This higher, encompassing quality is the bechinah of the sukkah, which surrounds a person with Hashem's protection from beyond his own striving.