Reasons and preventive measures for hollowing or decrease in bond strength after tile bonding:

1.  Substrate quality, product expiration date, water-to-cement ratio, etc.: First, check if factors such as substrate quality, product expiration date, and water-to-cement ratio are qualified.

2.  Insufficient curing time during pasting: Ensure that pasting is done within the curing time. During pasting, press and rub to ensure compactness of the tile adhesive.

3.  Re-adjustment after the adjustment time: If re-adjustment is needed, remove the tile adhesive first, then re-apply the mortar for pasting.

4.  Insufficient tile adhesive usage: During pre-laying, ensure accurate mortar usage. Adjust front and back distances using tapping methods, and the thickness of the tile adhesive should not be less than 3mm.

5.  Hot and dry weather: In hot and dry weather, reduce the area of each batch of mortar application. If the mortar appears to lack viscosity when touched, scrape it off and re-apply.

6.  Insufficient thickness of tile adhesive: If the thickness of the tile adhesive is insufficient, apply additional mortar promptly.

Note: Once the tile adhesive has exceeded its working time and hardened, it should not be re-used after adding water or other substances to it.