There is a setting in Timeslips under Setup > General that specified how many images per client are kept, the default is usually 12. Once it gets to that number it deletes the oldest to save new. You can change it but that will only impact going forward, not backwards. If you need an older bill your best option is to create a history bill. This is usually found under Reports > Billing. You can limit it by date, etc and by use of the right layout and appropriate options it will come close to your original bill.