what is the reason for data not entering the journey in sfmc while data entered in sfmc synchronized data extension?

Question

1. When creating a record in salesforce data synced in synchronized data extension but not entered into journey

Answer 1

These two things are not connected. You can have records coming into your synchronised data extension and not triggering a journey builder entry event and vice versa.

You should not look at synchronised data extensions at all, when debugging journey entries using Salesforce Data Entry Event. You should rather validate the entry criteria, and ensure the records being created/updated in Sales Cloud match these criteria.

There are numerous resources covering debugging Salesforce Data entry events, e.g. this, this and this.