can't move, to falter
趑 centers on being unable to move forward or hesitating in motion: faltering, stumbling, or being stuck in place.
趑 combines 走 (running) with 次, which likely contributes the sound and may relate to stopping or hesitating. The exact historical development is uncertain.
The running enclosure 走 surrounds 次, which looks like a person stuck in second place. Imagine trying to run but faltering and being unable to move forward.
For シ, picture a runner who can't move because she is stuck in thick syrup: she -> シ, and the syrup makes her falter.