The ONLY reasons why Venezuelan oil production went down are US boycott and sanctions + oil price collapse since 2014. There are other minor factors (that alone do not justify the numbers though), like the corruption of several people at PDVSA, but that's also being addressed with several investigations and arrests.
Oil production is a business that depends on access to credit: if you cannot get loans from banks it's almost impossible to support the high flow of investments needed to keep production up. Venezuela (which is not in default) has been cut off US banking system and that means:
- No access to loans, like anybody else.
- No access to transactions in USD, like anybody else.
The Venezuelan government has been working around this in the last years in various ways, which include diversification of the economy and new trade partners. Oil production will go up again, but it will take time. The rise in oil prices in the last months will certainly help.
Considering the fact that Trump has promised tougher sanctions recently, the future youre talking about might be really distant. Thats what analysts say( not only American though ).