So you do lab work for dentists? If you post your website or some other info it might be easier to help you with the issue.
If you want to improve cash flow you can either reduce expenses or grow revenue. My experience is that it is usually way easier to grow revenue since expenses are so thin for companies like yours unless you have wasteful habits.
Figure out who your ideal customer is and develop a marketing plan. Make sure the marketing plan uses multiple marketing medians to get brand recognition with the target market. Follow up with phone calls and offer test orders or test lab work. Make sure you can demonstrate a clear advantage to working with you compared to others.
Any clients you work with should respect your professionalism to sign an agreement with their orders to pay within a specified time frame. The agreement might also clearly spell out how the collections process works. Get signatures on orders, if they can order online include info like this in your TOS.