This is the first question I ask anyone who asks me if I can help fix their strategic planning. The reason is simple - every process has strengths and weaknesses, and the issues you are having with your strategic planning may well be the direct result of the process you chose to use.
Surprisingly, a common answer to this question is "We didn't really use a process" or "We read a couple of books and came up with our own process". Obviously, both of these can lead to problems. There are inevitable pitfalls in process design in strategic planning, and no process, or a mishmash of elements from several processes, can get you into those pitfalls quickly. So my first point is use a strategic planning process! Ideally, you want to use a process that's been tested and refined through use in thousands of companies in many different industries over the past 25 years, with a proven track record. Anything else is probably a mishmash of other processes put together by an inexpert strategist who wants to get into the business, and just as likely to lead to problems. Your best bet, of course, is a program like Simplified Strategic Planning, which is the most popular strategic planning model in use today.
The second point I want to make is that many processes that people use for strategic planning leave huge gaps where there should be data, analysis and documentation. My favorite example is Balanced Scorecard, which is an blown-up version of the Measures of Performance we started using in 1981. It's not a complete planning process - it's just a part of the process - and yet many companies treat Balanced Scorecard as their main strategy effort. That's like trying to drive from New York to LA by watching the speedometer and gas gauge - but not a map. Sure, you'll make good time...but where the heck will you be going?
The most common gap I find in other people's planning processes is implementation. Be sure to ask about how a process handles this, because implementation is the most common issue with strategic planning among attendees at our Simplified Strategic Planning seminar.