We read at the end of Pekudei that Moshe found himself unable to enter the completed Mishkan because of the cloud, representing the Shechina, which enveloped the tent (40:35). The Ohr haChaim points out that this seems to contradict the close of parshas Mishpatim where we read, "Vayavo Moshe b'toch he'anan," Moshe had no problem entering into the cloud which enveloped Har Sinai. Why the difference?
Ohr haChaim offers two answers: 1) Moshe had been called to enterinto the cloud that surrounded Sinai; he was not yet called to enter the cloud surrounding the Mishkan; 2) the cloud enveloped the Mishkan; the cloud hovered above Sinai, but did not block Moshe's access to the mountain.
I would like to suggest a third possibility which also relates to how we deal with the clouds that muddle our ability to move forward or to see things clearly. The difference between the two clouds is the context in which they occur. Sinai represents the experience of matan Torah; the Mishkan represents avodah, tefilah. Torah provides direction through the cloud -- chayav or patur, mutar or asur, the halacha provides a resolution, an answer, which allows us to move forward. Tefilah operates very differently. It's when we have given up the quest for answers or when we realize that despite our best efforts, we will fall short, that we turn to tefilah. Tefilah is not about finding a way through the cloud; it's about asking Hashem to just remove the cloud entirely because if we dare venture into it, we will be lost and be unable to find our own way out. We wait outside the cloud, not daring to enter, waiting expectently for a response.