It is not uncommon to struggle to load your first speargun and suspect it has come with too short a rubber. If this is you, the chances are you are not doing it right - speargun loading is 80% technique and only 20% strength.
Before we start lets get a few safety points first:
1. Never ever load your gun out of the water. To practice pulling back the rubbers, note how far back the notches are and remove the shaft to prevent accidental loading.
2. Modern spearguns must be loaded on your chest not your hip.
3. Generally the higher up on your chest you can get the handle the easier it will be to pull the rubbers the rest of the way.
4. It is easier in the water with a wetsuit on.
If you are still struggling then you may need to lengthen your rubbers. Bear in mind that the rubbers your gun came with are the optimum for performance, so changing them will compromise some accuracy and/or power. Putting longer rubbers on for a few months to get the technique down then putting the shorter ones back on may be a good idea.
Another option, particularly on longer guns with single 20mm rubbers, is to put two extra long 16mm rubbers on. This will make it much easier to load (although it will take twice as long) but will maintain the guns power.