Включи логи фпм
Если у ты не хостер, то стоит поставить себе process manager static нагрузить себя запросами и подобрать pm.max_children так чтоб на бд и фронт у тебя хватило памяти
Почитай, что тебе в дефолтном конфиге фпм пишут
; Error log file
; If it's set to "syslog", log is sent to syslogd instead of being written
; in a local file.
; Note: the default prefix is /var
; Default Value: log/php-fpm.log
error_log = /var/log/php5-fpm.log
; Log level
; Possible Values: alert, error, warning, notice, debug
; Default Value: notice
;log_level = notice
; Choose how the process manager will control the number of child processes.
; Possible Values:
; static - a fixed number (pm.max_children) of child processes;
; dynamic - the number of child processes are set dynamically based on the
; following directives. With this process management, there will be
; always at least 1 children.
; pm.max_children - the maximum number of children that can
; be alive at the same time.
; pm.start_servers - the number of children created on startup.
; pm.min_spare_servers - the minimum number of children in 'idle'
; state (waiting to process). If the number
; of 'idle' processes is less than this
; number then some children will be created.
; pm.max_spare_servers - the maximum number of children in 'idle'
; state (waiting to process). If the number
; of 'idle' processes is greater than this
; number then some children will be killed.
; ondemand - no children are created at startup. Children will be forked when
; new requests will connect. The following parameter are used:
; pm.max_children - the maximum number of children that
; can be alive at the same time.
; pm.process_idle_timeout - The number of seconds after which
; an idle process will be killed.
; Note: This value is mandatory.
pm = static