Slippage

I have an ea system that works most of the time.
Some time the orders aren't made.
Could this be a problem with slippage?
Like mt4 tries to enter but because of the slippage doesn't enter?

I got 3 pips slippage.

Increase your slippage to 1000 and see.

What errors are in the Journal? This would help a lot in resolving your problem.