If you can incorporate asking for referrals into the sales process, you will have a rigorous method for measuring and monitoring how many referrals are generated and what the conversion rates are for closing client-referred business.
Energy goes where attention flows, so without specific attention given to the act of asking for referrals, you may be coming up short in this area. For many salespeople, asking for referrals can be uncomfortable. Perhaps they feel unsure about how to do this effectively or they aren't confident they will get the desired response.
If you don't know how to do something and you believe that it may damage your existing relationships, then you may be inclined to avoid it all together. Additionally, if you make the common mistake of asking for referrals too early in the relationship, this can result in a refusal, which can erode your confidence.