How can we overcome decrease in sales if we didn't know the reason behind it?

we must know the reaso then we can solve the problem 

To know the reason is important and therefore start reviewing the sales cycle from scratch with an objective to find the reasons and to take corrective measures. 

For any business, a sales slump is an unwelcome but not necessarily unusual occurrence. The competitive environment is constantly changing, with new competitors entering and existing ones getting stronger. When you see that your business revenues are falling below forecast, you must decide whether taking action, such as a change in strategy, is warranted. There are few measure that could get the sales back on track.

Step1

Identify the reasons for the slump. Competitors might have lowered their prices, for example, drawing business away from your company. Consumer tastes or needs might be changing.

Step2

Determine how serious the problem is. A one-month drop in sales might not be a major cause for concern, particularly if the problem is related to a one-time event such as bad weather keeping customers from your store.

Step3

Change your marketing message. Consumers tune out messages they have heard over and over. Inject excitement into your marketing campaign by presenting new products or services you are offering and enhanced benefits to existing products or services.

Step4

Tap into new target markets. You might have reached your full revenue growth potential with the target markets that you concentrate on now.

Step5

Use existing customers to bring in new ones. Your most loyal and most satisfied customers can be effective salespeople for you if they tell friends and associates about your company. Offer them an incentive to bring you new business.

Step6

Change incentive compensation for your salespeople. Create a one- or three-month sales contest. Offer exciting prizes such as travel, dining certificates, golf greens fees, a day at a spa or cash bonuses.

How can we increase decrease in sales if we did not know the reason behind it? Follow the rating as a helpful

first you must investigate the reason, if it is very hard to find then you have to track your sales - gate to gate - and the most important is the field visits which well definitely lead you to find the reason once you got it it is solved.