The problem: You are well planning your time but you still get a lot of unexpected work.
I imagine you are scheduling all your to dos in your calendar to ensure that you do not plan for more time than available. I further imagine you save some extra time for unexpected problems in your tasks and that you are using the 60 % rule (planning only 60 % of available time to have rest for unexpected interruptions). But it happens that you get a lot of unexpected additional work so you have already reduced to plan only 50 %. Should you plan only - let's say 10 % of your time keeping the rest for the unexpected? - A clear "No" - this is the argument of all people who are against time planning. They say that everything is unexpected so you can't plan.
The reason: You are not alone - your planning depends on others planning!
Only a few tasks depend on you only so you can plan them as you want. For most tasks you require the cooperation of others. If your customers are acting in an unexpected way then you will be affected by their behaviour.
The solution: Get the others in the boat of proper time management!
Teach others about time management and plan with them. Ask them in advance what they want to achieve in the next year, in the next quarter and in the next month. - If you ask them in time to find out what they are going to do in the (near) future that might affect your own work and planning then you can save a time slice for them. Asking others for their plans and talking about their long-term desires you can get them to planning even if they never did before.
Planning does not bring only the advantage of getting an adequate amount of time - you have the possibility to split the issue in a "thinking about phase" and an "implementation phase" getting a better quality of the final solution.
Some people are more resistant against planning. They always have excuses why they can't tell you things in time and always know reasons why they can't plan. The only way to get them into the boat is to let them wait! They will find out that they will get served earlier and with better quality when they get to you in time.