May not apply to you, but with a lot of people who are good at getting starts and then get stuck, the problem is that they have a limited/idiosyncratic gameplan, that either works against some bowlers but not others, or works for a while until the captain figures it out and changes the field.
So for example, I know one bloke who is fine against pace but hopeless, totally hopeless against spin, so he might bat nicely for 12-15 overs and make 30 odd, and then the spinner comes on and he's toast. So unless the opposition don't have a spinner, he's pretty limited in the amount of runs he can score.
Another lad only has 2 shots, a cover drive and a sweep. He quite often hits 5-6 boundaries with these shots and gets to about 30 odd, and then the opposition put fielders to cut off his areas and after a few maidens he gets out trying desperately to play a different shot.
Other people simply get bored, or decide they're going to accelerate and overdo it.