it would help if you told us what your program looks like at the mo.
1, not every day, muscles need time to recover.
2, try wide grip pull-ups when you can do more than 20 add some weight.
If you dont have a weight belt for chin-ups you can put some weights in a rucksack.
If you do lat pulldowns on a machine, I would suggest changing to chins anyway, at least for 70% of the time