Light Yagami gets back his memory when he touches the death note. But Misa does not get back her memories when removing touches her with a piece of paper from the death note. Why?
In short: Unlike Light, Misa never owned that death note.
Now the relevant timeline:
Ryuk dropped a death note that became Light's - the one with the rules so we call it the rulebook. Rem handed a death note to Misa - without written rules so we call it the blank book.
Light and Misa met and touched each other's death notes. At this point, they were both able to see either shinigami. Also, Misa allows Light to keep the blank book but does not transfer ownership.
Misa gets caught and tortured by L and forfeits the ownership of the blank book so it is owned by Rem or the first other human to touch it (aka. Light).
At this point, Light is the owner of both death notes and knows that whenever he touches or get ownership of one of them, he regains his memories. Misa however only ever held ownership of the blank one.
Light makes the Shinigamis switch the death notes so that the blank note is now accompanied by Ryuk and the rulebook by Rem. Light hides the blank book and gives the rulebook to Rem.
Than Rem returns to the Shinigami's world searching for someone to give the rulebook while Lights gets himself incarcerated before finally giving up the ownership of the blank book (to Ryuk who returns to the shinigami world).
Rem finds a human who would abuse the death note (Higuchi) and hands the rulebook over to him.
Rem finds Misa in trouble and wants her to know that Rem would support her so she makes Misa touch a snipped of the rulebook - that is accompanied by Rem but never was owned by Misa so Misa gets the ability to see Rem but does not regain her memories.
After they defeated Higuchi, Light touches the rulebook and temporarily regains his memories so he kills Higuchi and assumes ownership of it, thus permanently regain his memories. After meeting again, Light orders Misa to find the blank book (that has been originally been given to her by Rem) so that Misa would finally regain her memories too. Note that since the shinigami swapped the notes, Ryuk now accompanies the blanc book.
If Misa would have remembered L's name, she'd have killed him right away but since she didn't, she makes the deal with Ryuk again what Rem would never have allowed.
Since Rem is connected to Light and sees the task force haunting Kira (Misa), Rem kills L and Watari as she realizes that this is the only way to safe Misa from being captured and sentenced to death.