1. Today, we received a complaint from a customer. (Added comma and "received" is more formal than "got", though "got" is correct in informal English.)
2. According to the customer, they cannot install our products on their machines because the holes on our products are a little smaller than what they demanded.
3.They can install our competitors' products on their machines very easily (smoothly?). (Both work, but I'd prefer "easily". You might want to say instead, "They can easily install our competitors' products on their machines.")
4. We researched our products but found out that there was nothing wrong with them. (Fine)
5.In addition to it, it turned out that the specification of the products, which are complained now, is totally same
as what we have supplied for the customer for a long time and they have never complained until now. ("which are complained now" is unclear. I'd try this:
"While the customers are complaining about the current shipment, we have determined that the specifications of the current shipment are exactly the same as the products that we have routinely supplied. They have never had any issues with that product in the past.")
6. In spite of it, it is very weird why the customer cannot install easily only the products which we recently supplied on their machines.
"We consider it strange that the customer cannot easily install the recent shipment since, as we said before, it has the same specifications as all previous shipments."