should net amounts for employees paycheques change half way through July? Why is this happening?

May I ask if there’s a recent update to the employee's profile, such as a newly set-up deduction or an updated pay rate?  

To help us pinpoint the cause, before you approve the payroll, click Preview payroll details to double-check the paycheck details. This will let you see which deductions or rates have changed compared to previous pay periods.  

If everything looks correct on the employee profile, but the amounts are still off, this could be related to the CRA Payroll Deductions Formulas.

Let us know in the comments if you have follow-up questions.