What do you think are the major management issues to keep a strategic plan successful?

If the organization gets off track, the leaders need to get it back on track. We also need to see if the plan is working. One of the best tools is to set check points or mileposts along the way. 
If an organization says it plans to increase its market share by 5%, for example, it needs to measure where it stands at various points in the plan. For example, where does it stand after year one? If the plan is to grow 5% in five years, perhaps it should achieve 1% after one year. This is not an automatic because the market may be hard to shift. So, market share growth might be scaled to be higher in early years or later years. The key idea is to set a reasonable milepost and measure it. 
So, if 1% growth for year one is the goal, the leadership needs to measure the progress. If the organization falls short of its goal, corrective action must be taken. The action may involve more resources than originally planned. Or, perhaps the organization did not commit the money and people it planned to use on its new strategy. Maybe the assumptions were wrong or the competitors stepped up their efforts. For example, we planned a 1% market share increase this year and a new competitor entered the market at a lower price. So, we will need to adjust our assumptions and perhaps change the time frame.