For the 1st working set, the weight chosen has to be such that you are able to complete 7 reps max. Then use that weight and leave a couple of reps in the bank. Don't train to failure ever! Doing so will tax your Central Nervous System unnecessarily. Failure begets failure.
Next, rest 60 - 120 seconds. Lower the weight by 10% so you can still get your 5 reps. Repeat the exercise, again leaving a couple of reps in the bank.
The key is to do as many sets as possible.
What lifting tempo do you use? Are you doing the deadlifts explosively?
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"In my experience, if you stay with the essentials you WILL build a repeatable swing undoubtedly. If you can master the Imperatives you have a champion" (Vikram).
The reason you can't sustain the lag is because you are so eager to make the club move fast (a reaction to the intent of "hitting it far"). So on a full shot you throw it away too early, which doesn't happen for your short chip. (bts)