This issue depends on how your limit order is placed.
If you're placing limit orders well ahead of time, getting filled is much more of an assurance, as it increases your book order.
Obviously, that involves a larger time frame. I find that on very large time frames, with larger SL and PT's, that you can almost always get filled, as long as you're willing to stomach price moving away quite a bit.
If you're scalping, this isn't a luxury for you, but you should be able to get some nice positive slippage for sudden strong moves that should offset either some of your non-fills or your "move aways" with whatever protocol you take.