The signal has to be Strong and it also has to be good Quality. You may be getting a strong signal now that your local transmitter has increased its power but the signal may still be low quality. That is, it may be suffering from dropout or interference or reflections etc.
Unless your television set itself is broken then the only reason that you are losing blocks of channels is because you have a poor signal at the set's aerial input. Re-tuning the Freeview may only change the group of missing channels. Tuning of the analogue signals won't make any difference.
Most television sets have a "meter" to show signal Quality and Strength. You can usually find it by working through the set up menu tree (check the set's handbook). This will show you what the problem is. You need about 70% of maximum signal quality and strength to guarantee good performance.
The poor signal could be for any of several reasons:
1) The aerial is misaligned: either twisted, not pointing directly at the transmitter mast, or is drooping. An aerial in this condition is very common.
2) The aerial cable is not properly connected inside the plastic junction box at the aerial itself (corrosion is very common in these things); or the wire in the plug is loose.
Boosters can only improve signal strength, they don't clean up poor quality signals. So don't waste your money on one.