Customers are quite defensive these days and are almost expecting staff to pounce on them; so their natural reaction is to refuse a drink and be dismissive. Naturally this puts you on the back foot and understandably you feel awkward to re-approach.
The key is to always look busy, but approachable. I recommend having a cloth in your back pocket so as soon as you see a customer walk into store you can start cleaning the surfaces. You must acknowledge them immediately by a simply good morning or afternoon. After approx. 45 secs. I suggest approaching them and introducing yourself and saying the following: “I’m just making myself a coffee, what would you like tea or coffee?”
By asking it this way I guarantee you’ll get more customers agreeing to a drink, therefore resulting in staying in store for longer and more receptive to discuss their requirements.