I've noticed when I practice with a metrenome, when I play open notes or the same notes its easy for me to get the evenness for each note. But when it comes to playing something like a scale, I wonder why things get kind of uneven. Are there any tips to help fix this?
Your left hand is not synchronized with your right hand. I suspect, at a guess, you are leading with your left rather than your right. If you slow down and analyze, assuming this is the case, you will be fretting the note on the beat and then plucking the string ever so slightly afterwards. When what should be happening is the left hand fretting the note ever so slightly before the beat and the right hand playing on the beat (leading the musical rhythm). You need to think of your right hand as a continuous time keeping machine, and the left hand simply changes the sound of the string played.

Perhaps start with one string. Say the A string. Just pluck the open string to a metronome for a while until the right hand feels almost on autopilot. Now, make sure to keep that right hand going and don't change anything about it, while starting to bring in the left hand. The idea is to keep the right hand steady as a rock, and not to let it falter in response to what the left hand is doing. 
May also be that your picking is already in time, but your fretting hand is out.  gweddle's advice is spot on.

Observe ...
1/ whether the timing is ok playing 2 or 3 notes of the scale pattern over and over
2/ whether the problem arises when you change strings.