Colin Busby, chief executive of Kier, said a concrete subcontractor, CV Buchan, had been recalled to provide additional support to beams at the centre.
Busby said: "We called CV Buchan in over a weekend about three weeks ago to put in props to support various beams. There has been some under-design on the car park but CV Buchan has agreed to carry out and pay for the additional works."
The mall does not open formally until later this year, although some shops have begun trading. The car park, which has several levels, is not open to the public.
Busby said the project had been behind schedule and a programme of works was under way to rectify problems and complete the handover to the client.
He was unclear how long the scheme would be delayed, although the project team would be working nights and weekends to make up time. He refused to be drawn on the additional costs this would incur. A Kier spokesperson said the additional work on the car park would not delay the project.
The programme was delayed, and the car park problem has just turned it into the job from hell
However, a Kier insider said the Castlepoint project had been a "nightmare" and that Kier would make little money on the job. The insider said: "The shopping centre construction programme has been delayed for some time because of technical difficulties. The car park problem has just turned it into the job from hell." CV Buchan was unavailable for comment.
Castlepoint, which was formerly known as the Hampshire Centre, is a 60,000 m2 mall. The developer is a limited partnership between Standard Life Investments, Castlemore Securities and Eagle Star Life Assurance.
In a statement the developer said: "We are clearly in active discussion with Kier to ascertain the full extent of the problem and how it has arisen. But at this stage the limited partnership is not aware of any potential delay in the anticipated completion of the development programme.
"As a purely precautionary measure temporary works have been installed to secure the affected areas, although the car park and the rest of Castlepoint remains open for trading as normal."
The £65m scheme is arranged around a V-shaped terrace of units, with composite cladding and decked parking. The project was to have been carried out in 107 weeks, with completion this autumn.