Complex sentence: The 1960s and 1970s saw a second population boom, caused by immigration from poorer regions of Spain: however this was not matched by construction of the necessary amenities and it was only in the 1990s that public investment resulted in additional schools, leisure facilities and housing.
Split into simpler sentences: The 1960s and 1970s saw a second population boom, caused by immigration from poorer regions of Spain. However this population boom was not matched by the construction of the necessary amenities and it was only in the 1990s that public investment resulted in additional schools, leisure facilities and housing.