First, you use the word "sinking", are you saying the lawn has sunk in some areas? Second, have you, or anyone, been walking on the grass during the winter, or playing on it? Third, when did you last cut the grass - if recently, when did you do the last cut before the winter?
Regarding re-seeding, its been either too wet or too cold for seed to do much, and it doesn't sound as if you're preparing the patches for seed properly either. Let me know the answers to the questions above, and then I'll give you some more advice.