Make sure you have set the fair price for smart СРА.
Make sure postbacks are delivered and the number of events matches them.
Make sure your domains are not blocked.
Make sure that the page content has not been changed critically (if a new page gives a CR much lower than the previous one, then there is a risk of the campaign to be auto stopped).
If you are sure that there are no technical problems with none of the points above, then feel free to experiment with new creatives in the campaign. When adding a creative that will give a higher LR (lead rate), the campaign speed will increase.