My issue usually occurs on 10 pins. When I start missing them its because of timing(too fast)  and tension in my arm swing.

When I slow my speed to the same as my strike ball, (form,  feet,  and arm swing) and relax my arm my spare percentage goes up.

If possible,  video yourself to see if you are standing and throwing where you think you are during practice. Video has been very helpful for me.

Since a video would only really help if you also have one of you before the troubles started, you might help yourself by reviewing your own delivery for spares balls.

For instance, if you usually hook the ball & have an inside out swing, throwing straight at left sides spares, for a righty, requires different body position & and a swing that is straighter.

Are you getting the ball into the swing (used to be called a "pushaway") at the right time to be in synch with your foot work? If this motion is later than your strike delivery, it could be causing you to pull the ball inside.

Are you aligning your shoulder and hand with your target line?
The ball goes where your body sends it, not where your eyes are looking.

Is your spare shooting footwork different from your strike footwork?
Review what you do, when you're doing it right.

I often suggest that we are, many times, but not always, our own best coach. Unless you know what you do, when you do it right, it's hard for yourself, no less anyone else to see what you're dong wrong.  If you can afford to have a regular coach, then they can help  you, but most of us can't. It is rare we have friends with sharp enough eyes and who know our game well enough to help diagnose it.

You should know at what point you get the ball moving, in relation to what step and what point in that step. (It could be when you first start the step, in the middle of that step or at the end of the step.)

You should make sure you start at the same spot on the approach in all houses. Don't assume that the 1st and 2nd set of dots on the approach are the same distance from the foul line in all houses. They aren't.

Sometimes it's helpful, when you're throwing a straight ball, to target closer to the pin or area you need to hit.

Make sure you post even spares shots. If you miss, you know where your feet are (where you slid)  and where you laid the ball down. This can be helpful in diagnosing the problem.

My biggest problem is left side spares. And I am right handed. I had a teammate give me a tip (spare system) that brought up my left side spare percentage to where I feel it should be, but it isn't working for me anymore.

What I found over time to use 'spare systems' as a guild, and to develop my own based on how I most comfortably target on the lanes. We all have different foot drift and ball roll, so what works for one will not translate to another. Spend lots of time doing nothing other than practicing different spare combinations from different angles. Drill down into what you're most consistent with and document every foot position and target.
 
If you can't hit your mark consistently, you need a coach to help straighten things out. I see lots of bowlers who strike because of forgiving THS characteristics, but struggle when they have to throw straight with accuracy.

Agree with Steven on Spare Systems. Use as a Guide! Spare Systems are good for a starting point. Everyone is different.  When you are practicing, video is good, but also, write down some notes on where you are standing and what is your target.

The key is finding your starting position on the approach and your target line on the lane that is going to give you the highest percentage. Everyone is different. Once you figure things out, not only will your scores improve, your confidence is going to improve.

I really like Charlest statement: The ball goes where your body sends it, not where your eyes are looking. 
 

Before I started using a spare ball, 4 pins and 7 pins were tougher for me than 10 pins.  I cant count how many times at nationals me or my team mates would hook past one of those early on, and skid past later.  Biggest advantage of a spare ball to me was left side spares and right side combo spares.

Well I did not bring my pitch black out of the bag to throw at spares, but I did make sure I didn't squeeze the ball and throw what felt like my normal strike ball at my spares. For right side spares, I did kill the ball by coming straight up the back. The only spares I missed all night were splits caused by the ball coming in light due to carry down. I covered every makeable spare and never threw what felt like a bad spare shot. I think my whole problem was just mechanics