1. Double check your filament. Are you trying to print with ABS filament on Jr.? ABS filament require heated bed.
2. If you are indeed using PLA filament, use blue tape. The one people use when they paint house. It do come with different color, but for reasons unknown, the blue color stick best.
3. double check your z offset, the nozzle should be 0.1mm-0.2mm above the bed. if space is 0.25-0.4mm, it will print normal in the beginning but unable to stick later. if space is smaller than 0.1mm, you will have trouble to take your print off bed.
4. change blue tape every 4-5 print. also make sure soap wash your hand when apply the blue tape. sweet, lipid and grease on your hand will prevent PLA stick to blue tape.
5. google keywork ABS juice, glue stick, jello together with 3d print, many useful tips.
(Da Vinci 1.0, Jr. 1.0 RAMPS, miniMaker) X4, (Creality CR-10S, CR-10 mini, Ender-3) X4, Anycubic MEGA X4, Anycubic Chrion X1, ADMILAB Gantry X2 (MonoPrice Maker Select V2, Plus, Ultimate)X4--Select mini X1, Anycubic photon X4, Wanhao duplicate D7 X1.
iNSTONE Inventor Pro X2, CTC Dual X2, ANET-A8, Hictop 3DP-11, Solidoodle Press, FLSUN I3 2017X1