my modem loose lock all time

please help me out am using Idirect 3000 series am connecting to amc-12 satellite with a 1.8m dish to get signal is problem and even if i  manage to get it go off once i will have to stress hard to locate the right satellite
but after i had lock all my bolt and power on my modem it will go off 
i start all again even at that the signal is not up to the maximum signal that my isp want me to  get
with the isite software i can only mange to get 16.5 and it opten loose lock please i had tried all i know i had change the lnb feedhorn and buc but still the prblem persist
could it be that the modem that is cousing it sholud i re load the package files again 
please i need help am in Nigeria edo state

ESEgbue