The main difference between the two polymerases is that Platinum polymerase is a modified version of Taq polymerase that has a built-in hot-start mechanism. This means that the enzyme is inactive at room temperature and only becomes active after a high-temperature denaturation step, which prevents non-specific amplification and improves PCR specificity, sensitivity, and yield. Taq polymerase, on the other hand, does not have a hot-start feature and can start extending primers even at low temperatures, which can result in unwanted products and reduced PCR performance. Another difference is that Platinum polymerase is a "fast" enzyme that can synthesize DNA four times faster than Taq polymerase. This allows for shorter extension times and faster PCR cycles.

My advice is to consider some changes in your protocol, starting with annealing temperatures and increasing the elongation time.

Dear Naziya Nabi

in my experience, the standard Taq work well for small fragments up to 1-1.5kb, for longer fragments a hot start taq and if possible even an high fidelity is preferable

there are a lot of good taq suitable for this purpose

my preferred one is the KAPA HIhi Hotstart

but also other as

Takara long range

Pfu ultra fusion II

Platinum

Clone amp takara

work well with longer dna sequences.

best regards

Manuele