Adobe Illustrator has a useful function which aligns your work to the screen pixels so that you can develop screen graphics with sharp edges and corners. The feature is Align to Pixel Grid. As useful as this feature is it can sometimes cause problems if you are not aware that it is turned on.
Align to Pixel Grid adjusts the size and placement of vector graphics so that they will look good on the screen. When creating graphics for print or when you need exact placement of objects, Align to Pixel Grid can cause undesired interactions. If Illustrator will not place an object where you want it or if it changes the stroke width of your lines it may be a pixel grid issue.
To fix this, turn off the Align to Pixel Grid setting. This setting can be turned off for selected artwork or for all newly created artwork.
Turn off this feature for specific objects:
Select the objects.
Open the Transform Panel. Click Window > Transform (Shift + F8).
Uncheck the box at Align to Pixel Grid.
All of the selected artwork is no longer aligned to the pixel grid when you size or move it.
You may change all the artwork in the current document by selecting all artwork Select > All (CTRL/CMD + A) and then Uncheck the box at Align to Pixel Grid as above.
Prevent Illustrator from aligning all new artwork to the pixel grid in this document:
Uncheck the option in the Transform Panel flyout menu. Click the flyout menu control on the Transform Panel. This is found just below the close panel (X) in the upper right corner of the panel. On the flyout uncheck Align New Objects to Pixel Grid.
Prevent Illustrator from aligning all new artwork to the pixel grid in future documents:
Change the option in the New File dialog. Uncheck the option Align New Objects to Pixel Grid. By default Illustrator no longer aligns new objects to the pixel grid in files created with this option unchecked.