A person/thing that has a good start but fails midway through the journey

Solution 1:

The word is anticlimactic. It popped up in my mind as I recalled some famous sports commentary. Then checked the definition on web and the term aptly describes the situation I had in mind.

Here's OED's definition -

anticlimactic - Causing disappointment at the end of an exciting or impressive series of events.

‘it was an anticlimactic finish to the match’

Solution 2:

A damp squib is a similar concept.

OED - "a situation or event which is much less impressive than expected"

A squib is a firework, which promises exciting things, but then being damp it fails to work properly.

However, it turned out to be a damp squib as the local leaders and even party’s own MLAs remained absent from the programme. [The Times of India]