How retention works
Each payment certificate holds back a percentage of the value certified until the amount held reaches the limit in the contract data, usually a percentage of the contract sum. After that nothing more is held.
Part of it, commonly half, is released at practical completion (or taking over), and the rest at final completion or the end of the defects period. Your contract data sets the percentages; change them to match.
Worked example
On a R10 million contract with 10% retention limited to 5% of the contract sum, R8 million has been certified. 10% would be R800 000, but the limit is R500 000, so R500 000 is held. Half, R250 000, comes back at practical completion and R250 000 at final completion.