Maybe the varying effectiveness of Force telekinesis is a matter of how much concentration or energy they can spare at a given moment, or how much willpower is (consciously or unconsciously) resisting them. Maybe the Force has currents and fluctuations on a large scale.
This is the only explanation that makes sense to me. The parting words "May the Force be with you!" always indicated to me that Force powers can't necessarily be relied upon.
In the opening multipart series of the season, Anakin asked someone to guard him while he concentrated on telekinetically bringing down a building. He was vulnerable while he did this; perhaps the limited prescience which lets him block blaster attacks with his lightsaber was hampered.
If a Dungeons & Dragons model applies, and Force powers are something like psionics or spells, then they cannot be used in an unlimited way.