Missing item in order

Last week, I received my order with 1 item missing; it was a listing for 2 items but only 1 arrived. I contacted the seller right away and they confirmed that one item was missed. I asked them to resend the missing item and to send me the tracking number once shipped, and they agreed.

I waited 5 days and they still hadn't sent me the tracking number, so I messaged them again to ask. The seller said the missing item has been reshipped and I would receive the tracking number in a few day. It's day 8 now and I still don't have a tracking number.

Should I be worried that the seller is lying to me? Should I just start a dispute now?