Losing the Bar?
If the bar is slipping out of your hands on the deadlift or other exercises, your grip isn't strong enough. Note that you could have grip issues even if you don't feel yourself losing your grip. A weaker grip leads to less force transfer to the bar, which will limit the amount of weight you can move.
There are two ways to diagnose this. First, if you can use 10-15% more weight with straps than without straps, your grip is weak. Likewise, if you always pull slowly when the weights get above 80%, your grip is weak. Here's one way to boost grip strength very quickly: