Thicker specimens require more water between the cover glass and the slide. If too little water is used, then there is the real possibility that the specimen is sandwiched and squashed. Sometimes this is done deliberately in order to limit the movement of certain specimens. Water fleas, for example, can be immobilized this way. In other cases the pressure might be too high and the specimen is damaged.
If you need more space between the slide and the cover glass, then simply adding more water will not always work well. The cover slip then floats on top of the water and every small bump against the microscope or against the table will cause the cover glass to vibrate. A calm observation is much more difficult under these circumstances. What we need is a spacer which holds the cover glass at a defined distance over the slide. There are several ways of making such a spacer.
You can carefully attach small pieces of wax to the four corners of the cover glass. The wax should be first warmed by rolling it between your fingers into a small ball. Small pieces of soft wax are then carefully adhered to the four corners of the cover glass, which is then pressed against the slide, which does not yet contain the sample. If soft wax is not available, then one can also carefully dip the four corners of the cover glass into Vaseline. The wax or Vaseline will both adhere the cover glass to the slide and also increase the space for the sample. The liquid sample can then be applied from the side.
A second possibility is to use two additional cover glasses as a support. Place two cover glasses on a slide, slightly separated. Place a third one across these two, forming a bridge. You can then add your sample with a pipette. The water will be drawn in beneath the three cover glasses and hold them in place.
Last, it is possible to use slides which have a concave indentation. The amount of water that can be held by such a slide can be quite considerable and it may be difficult to observe if specimens float in and out of focus. Be aware that thick layers of water beneath the cover glass can significantly reduce the resolution of the image, especially for the higher magnification objectives.