Advice converting interviews to offers

Experienced software engineer here, using a throwaway account. Been in the field for a little over 7 years. Looking to change jobs. Started applying to companies (startups and some big cos) a few months ago. Got telephone interviews from 10 companies and 8 of those invited me on site. All of the phone interviews went really well and I could feel a great sense of excitement on the other end as well (or at least I feel so, given that 2 of them didn't want to move past the phone interview stage). The problem here being that once I go on site I am having trouble converting interviews to an offer. A typical on site interview has been solving questions on the whiteboard or working with engineers on some problem. In most of these interviews I can feel that I am able to provide good answers to questions posed. But there's a theme I've seen repeating with myself * Was able to really impress one or two interviewers * Answered questions with good level of technical detail * Failed to impress one of the interviewers Wondering if there are others that have had similar experiences and might be able to provide some advice?