Different voltage levels for LCD and microcontroller
Check that: 

\$V_{OH-µC}&gt;V_{IH-LCD}\$

\$V_{OL-µC}&lt;V_{IL-LCD}\$

If you're reading data from the LCD too, check:

\$V_{IH-µC}&lt;V_{OH-LCD}\$

\$V_{IL-µC}&gt;V_{OL-LCD}\$

Where :

\$V_{OH}\$ is the High level Output Voltage

\$V_{IH}\$ is the High level Input Voltage

\$V_{OL}\$ is the Low level Output Voltage

\$V_{IL}\$ is the Low level Input Voltage

Make sure to read these value in operating conditions as close to yours as possible. They're typically given in datasheet under different load and supply voltage conditions.