Hmm, This is a concern that you are having this kind of problem, imagine a 4 speed automatic transmission that will not go into 5th. You description is very unclear as to what exactly the trans is doing wrong. When you have it in D1, it will start in D1 and stay there. When you have it in D2 then it will start in D1 and shift to D2, then stay there.
When you have it in D or 3 (depending on vehicle model) it will start in D1 then shift to D2, then shift to D3 then stay there.
D4 or OD (depending on model) is the most used range and will shift normally from D1 through the ranges until it gets to OD and stay there depending on accelerator, load downshift if needed as the computer thinks is necessary.
I have no difficulty pointing the right direction for diagnostics but I need all the help you can give. Information is very much needed, even if you think it is unnecessary. Maint. History, repair history, mileage, add-ons (shift kit, air filter, radio, alarm, etc) can be very important. What have you done so far to find the problem or added to the trans to correct the problem? Post back and we can help more, Transman