• 1. can you explain as to why some pollen grains fail to germinate?
• can pollen grains of one plant species germinate on stigma of other species.give reason
• are all pollen tube of equal length,. if not why?
• do all pollen tubes reach the stigma

1)Some pollen grains fail to germinate as they don't get all the essential conditions for the germination of pollen.
2)No, pollen grains of other species do not grow.
3)Flowers receive all types of pollen which includes pollen of same species and of different flowers as well. This happens because of means of pollination i.e. butterflies, bees, wind etc. The stigma of flower receives different kinds of pollen, but pollen belonging to same species grows and survive and rest of the pollens fail to grow. 
4)No, pollen tubes are not of equal length. It is so because older germinated pollens will have longer developed tube as compare to newly germinated pollen which will have shorter pollen tube.
5)If pollen germinates on stigma, pollen tube will reach the stigma.