Answer: It would be easier to tell you why your trees aren't producing, rather than why they aren't blossoming. Blooming trees that don't produce have pollination problems which are usually easily fixed. However, trees not blossoming are a bit more perplexing, but there are a few possibilities. First let's consider their age. You haven't indicated whether they are standards or dwarfs, nor their variety. While either should be producing by 6 years old, I guess it's just possible that if these are standards, they could be just a bit slow, and may start blossoming this year or next.
How has the spring weather been? Have springs been unusually rough? If so, buds could be freezing. This is often a problem with growing fruit trees in the north. There are some trees which are specially bred to be freeze tolerant. Ask your local county agent for varieties that do well in your area.
You mentioned that they are planted closely with other trees. Hopefully, not too closely since fruit trees need good air circulation to resist disease. Plus, they all need to receive lots of sunshine. They will not blossom if they are shaded heavily. If the pears are dwarf and planted among standard or even semi-dwarf trees, there may be too much shade for the pears to do well.
Are you pruning correctly? Too little pruning and your tree could be too dense to blossom well. Too much and you could be stressing a large tree. While neither of these would cause the tree to not blossom entirely, correct pruning is essential for good fruiting.
Lastly, don't over fertilize your trees, especially pears which develop a disease called fire blight when given too much nitrogen. Use a fertilizer with high phosphorus (the middle number) which will encourage good root and bloom.
Hopefully you've found your answer in these suggestions. If not, contact the supplier of your trees, or your county agent.
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