Step by step solution
Weakly bonded tiles should be removed and the gap should be filled with a suitable repair mortar, like weberep 331 TX.
Methode of application
For such applications it is preferable to use a C2 grade tile adhesive such as
Webercol flex - webercol max – webercol plus – webercol premium – webercol fast – webercol F1 - etc
Mix and apply the selected tile adhesive with a suitable notched trowel.
For large tiles, only a flexible tile adhesive like webercol flex should be used.
Back-buttering is recommended.
If the substrate is non-porous (porcelain tiles, etc...), priming is highly recommended with webercol prime.